API Docs: convert

[steveklabnik]

Part of #29329

http://doc.rust-lang.org/std/convert/

Here's what needs to be done to close out this issue:

• the module documentation is great, but has a lot of abbreviations, so the wording could be cleaned up a bit.
• AsMut should compare itself to AsRef, and should show a sample implementation.
• AsRef needs far better docs, as it is one of the most misunderstood traits in Rust. Specifically it should talk about its relationship with Borrow. It also needs an example implementation.
• From needs a lot of work; it should talk about its usage in ?, it should show an example implementation, and general improvement.
• Into is mostly fine, but "generic impls" should say "implementations" and it should be above examples.

[steveklabnik]

I am happy to mentor anyone who wants to tackle this issue.

[maccoda]

I would be keen to give this one a go @steveklabnik I'm interested in understanding this section better.

[steveklabnik]

Awesome, @maccoda ! Let me know if I can help you help me in any way 😄