Elaborate on based_on_style origins and use cases. (#896)
Elaborate on based_on_style origins and use cases. (#896)

Several Google OSS projects I have contributed to have mistakenly assumed that they should use
the `"google"` style with `indent_width` manually set to `2`. Since Google does not follow the 
(public) Google Python Style Guide however, this creates numerous diffs between the internal 
formatter and `yapf --style=google`.

`yapf --style=yapf` does not perfectly match the internal formatter (e.g. for docstrings),
but it is much closer than `yapf --style={based_on_style: google, indent_width: 2}`,
particularly since it sets `SPLIT_BEFORE_EXPRESSION_AFTER_OPENING_PAREN=True`.
google/yapf,google/yapf