Skylint: report missing documentation for a function's return value

In addition to checking the function parameter documentation,
skylint now also checks whether the return value is documented in a
'Returns:' section in the docstring.

The same restrictions as for parameters apply:
- Private functions need no documentation
- If a function has a single line docstring, it need not document the
  return value.

In addition, I improved the docstring parsing:

- Previously, the beginning and end of a section (e.g. 'Args:',
  'Returns:') were determined by line breaks. Now, they're determined by
  indentation and missing line breaks are reported. This change should
  make the docstring parser more robust.
- Additional indentation is not warned against anymore.
  There are many situations where it makes sense, like example code.

Both of these changes were motivated by the results of the linter on
Skylark files "in the wild".

RELNOTES: none
PiperOrigin-RevId: 168660248