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

This is mainly in preparation for the eventual implementation of #![feature(nonpoison_rwlock)] (which I am planning to do soon), and the issue for that can be found in #134645. These changes should make it easier to find things in the code and port things over to a future nonpoison variant 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 Stabilize rwlock_downgrade library feature #143191

Edit: some more cosmetic changes to the rwlock test file, and it is simply moving tests up or down depending on if they test poisoning or not. This will again make the implementation of nonpoison_rwlock much easier.