- Explicitly mention that `AsRef` and `AsMut` do not auto-dereference
  generally for all dereferencable types (but only if inner type is a
  shared and/or mutable reference)
- Give advice to not use `AsRef` or `AsMut` for the sole purpose of
  dereferencing
- Suggest providing a transitive `AsRef` or `AsMut` implementation for
  types which implement `Deref`
- Add new section "Reflexivity" in documentation comments for `AsRef`
  and `AsMut`
- Provide better example for `AsMut`
- Added heading "Relation to `Borrow`" in `AsRef`'s docs to improve
  structure

Issue rust-lang#45742 and a corresponding FIXME in the libcore suggest that
`AsRef` and `AsMut` should provide a blanket implementation over
`Deref`. As that is difficult to realize at the moment, this commit
updates the documentation to better describe the status-quo and to give
advice on how to use `AsRef` and `AsMut`.

Fixed examples in sections "Generic Implementations" of `AsRef`'s and
`AsMut`'s doc comments, which failed tests.

Better conform to Rust API Documentation Conventions

Changed wording in sections on "Reflexivity":
replaced "that is there is" with "i.e. there would be" and removed comma
before "with"

Reason: "there is" somewhat contradicted the "would be" hypothetical.
A slightly redundant wording has now been chosen for better clarity.
The comma seemed to be superfluous.