Make the example use DoubleEndedIterator for map, like it said it would.

The `bound` argument in `std::sync::mpsc::sync:channel(bound: usize)` was not defined in the documentation.

rust-lang#30530

Remove exclamation marks from the the example error descriptions:
> The description [...] should not contain newlines or sentence-ending punctuation

Also, end sentence with a period.

Add missing examples to SocketAddrV6

r? @steveklabnik

cc @frewsxcv

Fix two small issues in iterator docs

- `collect()` is a regular method, not an adaptor (does not return an Iterator). I just randomly picked `filter` as a third common adaptor to mention instead.
- Fix example in `Map`'s docs so that it uses the DoubleEndedIterator implementation

Add a tracking issue for enum_set

I totally forgot this even existed!

Define `bound` argument in std::sync::mpsc::sync_channel in the documentation

The `bound` argument in `std::sync::mpsc::sync:channel(bound: usize)` was not defined in the documentation.

…enkov

Remove completed FIXME.

rust-lang#30530

Follow our own recommendations in the examples

Remove exclamation marks from the the example error descriptions:
> The description [...] should not contain newlines or sentence-ending punctuation

…eGomez

Document how lock 'guard' structures are created.