Rollup merge of rust-lang#41348 - lukaramu:std-path-docs, r=frewsxcv

Improve std::path docs

Fixes rust-lang#29368.

This PR contains a pretty significant redistribution of some of the module docs to more appropriate places, as well as general expansion, clarification, and additional examples.

For more details, see the commit descriptions.

r? @steveklabnik