Improve docs differentiating `Borrow` and `AsRef`

[pnkfelix]

Spawned of off #24140

This should be better documented, but there's definitely a reason that things are set up this way.
...
... it may be worth opening a separate issue for improved documentation and missing impls on AsRef.

[steveklabnik]

Notes from talking to @aturon

borrow is used in things like hashmap, where you have a key, and the map needs to store the owned version, but you want to look things up by the borrowed version. HashMap<String, i32>, but you want to look up by string literals: &str.

for most types, you want T and &T. but for things that take slices, there's a few different slice types. Borrow is good for this

it's more restrictive than asref, because the assumption is the borrow is the same wrt hashing or comparison: String, &str, and &String all hash to the same value.

AsRef: you can get a different reference out of this. that's it. a conversion, not an equivalence.

anything htat impls borrow should implement asref, but no blanket because of coherence