Part of #29329

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

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

• the module docs are written a while ago, and contain some questionableish advice. It's not actually bad, it's just not the way I'd orient all of this. A particularly enthusiastic contributor could toss this out and re-write it from scratch. If the rest of this work gets done, we can close the ticket without doing this, but it would be nice.
• Builder's docs are... okay. Just very uninspired.
• JoinHandle doesn't go into enough detail, and should show off, for example, the detach behavior.
• LocalKey could use some links and general cleanup.
• Thread has very little and very boring docs.
• panicking could use some more advice on when to use this.
• park should have its module documentation inlined here, and cleaned up.
• park_timeout could use links to park
• spawn needs a lot more docs generally. It's the main interface to this module!
• yield_now doesn't explain any of the "why".
• Result should at least show off the "use std::thread; thread::Result" pattern, like all module-specific Result types.