API Docs: std::ops

[steveklabnik]

Part of #29329

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

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

• almost all of the traits suffer from "stuttering", that is, their summary lines should drop "the X trait" and say "the X operator", with the symbol. Don't stutter in operator trait descriptions #40818
• Range needs a better summary/explanation distinction.
• RangeFrom has the same issue.
• RangeFull same
• RangeTo same
• All of these range traits could use examples of implementing them to let people pass ranges directly, with examples.
• Deref is a very important trait with little documentation.
• DerefMut should link to Deref, and vice versa.
• Drop is very important yet has little documentation.
• Fn has a bad summary and a lot of its explanation is inside of the example.
• FnMut same
• FnOnce same
• Some traits only have "prints when called" examples, while others have amazing, real examples. Improving these would be nice but isn't required to close this ticket.