Reoganize poison::rwlock file

[connortsui20]

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

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