`std::net::tcp` docs: Use current syntax and types

Doesn't touch non-comment lines. This changes various type_names to TypeNames
and fixes the example for `tcp::accept` that was still using the old
`match` syntax and `{|args| ...}` closures.
auto merge of #5185 : ben0x539/rust/net-tcp-docs, r=brson

This changes various type_names to TypeNames and fixes the example for `tcp::accept` that was still using the old `match` syntax and `{|args| ...}` closures.

The `accept` example was fairly outdated. I was just going to stay away from all the IO things until the scheduler revamp lands, but `accept` is probably one of the obvious starting points for networking stuff for a learner, and I don't want to get in the way of anyone's enthusiasm.

Doesn't touch non-comment lines, so I hope I will get away without learning about unit tests. It doesn't seem like the test system is set up to extract tests from doc comments right now.