DOC: `merge` doc for `indicator` parameter can be improved
#### Location of the documentation

https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.merge.html#pandas.DataFrame.merge


#### Documentation problem

The documentation is correct but hard to read, currently:

```... If string, column with information on source of each row will be added to output DataFrame, and column will be named value of string. Information column is Categorical-type and ...```

#### Suggested fix for documentation

I believe the following paragraph will be easier to read:

```
If True, adds a column to the output DataFrame called “_merge” with information on 
the source of each row. The column can be given a different name by providing a 
string argument. The column will have a Categorical type with the value of “left_only” 
for observations whose merge key only appears in ‘left’ DataFrame, “right_only” for 
observations whose merge key only appears in ‘right’ DataFrame, and “both” if 
the observation’s merge key is found in both.
```
