PY-16991 Don't mix up section headers and parameter references in Google docstrings

PY-16991 Don't mix up section headers and parameter references in Google docstrings

To avoid completion of headers instead of parameters we require at
least one space indentation inside sections in Google code style
docstrings. Thus parameters references are not inserted falsely and
we still can use the rule "if there is reference under caret don't
suggest section headers".

For Numpy we have nothing to do than to let headers be suggested
under sections (where parameters normally go), since it's not really
obvious how to distinguish incomplete parameter references from
incomplete headers there (thanks to its zero section indentation).