Cleaned up throughout std::path's docs
Cleaned up throughout std::path's docs

Part of #29368.
* added missing links
* updated method summaries to use 3rd person style
* added missing periods in `Component`'s variant summaries
* use standard iterator boilerplate in `Components`' and `Iter`'s docs
* added example to `Iter::as_path`, adapted from `Components::as_path`'s
  example
* consolidated examples for `Path::file_name`
* some other small fixes

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