This commit is a purely cosmetic change to the documentation and
ordering of items in the `rwlock.rs` file, which will help discern the
actual difference between the `nonpoison` and `poison` variants of
`rwlock`.
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 stabilization PR #
143191
d710a8e8d7