BUG: the pd.to_timedelta()  incorrectly parses the 'M' as minutes.
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

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#### Code Sample, a copy-pastable example

```python
>> pd.Timestamp.today() + pd.to_timedelta('1 M')

```

#### Problem description

The documentation in version 1.0.5 is described as follows:

> Possible values: (‘Y’, ‘M’, ‘W’, ‘D’, ‘days’, ‘day’, ‘hours’, hour’, ‘hr’, ‘h’,... 

Since in such a particular order, it should be understood as year, month, and week. 

But running my code gives the result of a minute for 'M'. However, 'Y' produces the correct parsing result.

I've seen some discussion there about parameters optimization, and whatever the outcome of the discussion, you shouldn't continue to place this bug here, please either change the function or change the documentation.

And perhaps you might consider separating _to_datedelta_ api to reduce complexity. 

Thanks.
#### Expected Output

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 3.10.0-1127.10.1.el7.x86_64
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.5
numpy            : 1.18.1

</details>
