DOC: use substitution decorator for business month classes (#25828)
- [X] closes #25828 
- [ ] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry


Validation script
`CustomBusinessMonthEnd`
```
################################################################################
########## Docstring (pandas.tseries.offsets.CustomBusinessMonthEnd)  ##########
################################################################################
                                                                                
DateOffset subclass representing one custom business month, incrementing        
between end of month dates.                                                     
                                                                                
Parameters                                                                      
----------                                                                      
n : int, default 1                                                              
normalize : bool, default False                                                 
    Normalize start/end dates to midnight before generating date range          
weekmask : str, Default 'Mon Tue Wed Thu Fri'                                   
    weekmask of valid business days, passed to ``numpy.busdaycalendar``         
holidays : list                                                                 
    list/array of dates to exclude from the set of valid business days,         
    passed to ``numpy.busdaycalendar``                                          
calendar : pd.HolidayCalendar or np.busdaycalendar                              
offset : timedelta, default timedelta(0)                                        
                                                                                
################################################################################
################################## Validation ##################################
################################################################################
                                                                                
9 Errors found:                                                                 
        Summary should fit in a single line                                     
        Parameter "n" has no description                                        
        Parameter "normalize" description should finish with "."                
        Parameter "weekmask" description should start with a capital letter     
        Parameter "weekmask" description should finish with "."                 
        Parameter "holidays" description should start with a capital letter     
        Parameter "holidays" description should finish with "."                 
        Parameter "calendar" has no description                                 
        Parameter "offset" has no description                                   
2 Warnings found:                                                               
        See Also section not found                                              
        No examples section found                                                                                                                   
```

`CustomBusinessMonthBegin`
```
################################################################################
######### Docstring (pandas.tseries.offsets.CustomBusinessMonthBegin)  #########
################################################################################
                                                                                
DateOffset subclass representing one custom business month, incrementing        
between beginning of month dates.                                               
                                                                                
Parameters                                                                      
----------                                                                      
n : int, default 1                                                              
normalize : bool, default False                                                 
    Normalize start/end dates to midnight before generating date range          
weekmask : str, Default 'Mon Tue Wed Thu Fri'                                   
    weekmask of valid business days, passed to ``numpy.busdaycalendar``         
holidays : list                                                                 
    list/array of dates to exclude from the set of valid business days,         
    passed to ``numpy.busdaycalendar``                                          
calendar : pd.HolidayCalendar or np.busdaycalendar                              
offset : timedelta, default timedelta(0)                                        
                                                                                
################################################################################
################################## Validation ##################################
################################################################################
                                                                                
9 Errors found:                                                                 
        Summary should fit in a single line                                     
        Parameter "n" has no description                                        
        Parameter "normalize" description should finish with "."                
        Parameter "weekmask" description should start with a capital letter     
        Parameter "weekmask" description should finish with "."                 
        Parameter "holidays" description should start with a capital letter     
        Parameter "holidays" description should finish with "."                 
        Parameter "calendar" has no description                                 
        Parameter "offset" has no description                                   
2 Warnings found:                                                               
        See Also section not found                                              
        No examples section found                                               
```