DOC adding return type and improving module doc
DOC return types in datasets.lfw
DOC numpydoc convention for Bunch-returning functions

Also cap lines at 80 chars, remove trailing spaces.

As suggested by @jnothman at
https://github.com/scikit-learn/scikit-learn/pull/3316/files#r14269818
Merge branch 'pr/3316'

Fixes #3316.