Rollup merge of #41376 - mbrubeck:docs, r=frewsxcv
Rollup merge of #41376 - mbrubeck:docs, r=frewsxcv

Expanded docs and examples for PathBuf::file_name and friends

This addresses some common surprises when `PathBuf::set_file_name` is called on the path of a directory rather than a file.

r? @steveklabnik

GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust