Cleanup rustdoc links

[xzfc]

This PR cleans rustdoc comments, primarily links. No code/API changes.

Don't use http/html links

Why: these are clickable in rust-analyzer and lintable by rust doc (rustdoc::broken_intra_doc_links):

• /// Wrapper for [`std::fs::read_dir()`].
  

• /// Creates a [`File`] from a raw file and its path.
  

• /// This struct is created via [`fs_err::read_dir`].
  

These are not:

• /// Wrapper for [`fs::read_dir`](https://doc.rust-lang.org/stable/std/fs/fn.read_dir.html).
  

• /// Creates a [`File`](struct.File.html) from a raw file and its path.
  

• /// This struct is created via [`fs_err::read_dir`][fs_err::read_dir].
  /// [fs_err::read_dir]: fn.read_dir.html
  

Other changes

• Fix typo: seek_wriite.
• Always spell full paths.
  • Use `std::fs::something`, not `fs::something`.
  • Use `fs_err::Something`, not `crate::Something`.
  • Why: extra clear because there are multiple similarly named structures.
• Refer to the crate as fs-err, not `fs_err`. That's how it's spelled in the readme.

[andrewhickman]

Thanks!