reorganize rwlock file

This commit is a purely cosmetic change to the documentation and
ordering of items in the `rwlock.rs` file.

List of changes (lots of small things):

- Clean up some of the existing field doc comments
- Add documentation for every field in struct definitions
- Consolidate related implementation blocks (1 implementation block per
  guard instead of 2)
- Use the lifetime name `'rwlock` instead of `'a`
- Reorder implementation blocks to be consistent across the entire file
  (follows the order `ReadGuard`, `WriteGuard`, `MappedReadGuard`,
  MappedWriteGuard`)
- Move simple trait implementations to the bottom of the file
- Rename the `poison` field in `MappedRwLockWriteGuard` to
  posion_guard`
- Cut off comments at 100 columns
- Update the documentation of `downgrade` to match https://github.com
  rust-lang/pull/143191

Author: connortsui20