[netstack3] Document lock_ordering module

Add a module-level doc comment explaining how the lock_ordering types
are used and why the lock ordering for Netstack3 is linearized.

Bug: 120973
Change-Id: Ie73081c47ff8090b5ad673d4a0be4a3d534f36a5
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/818588
Commit-Queue: Alex Konradi <akonradi@google.com>
Reviewed-by: Ghanan Gowripalan <ghanan@google.com>
Fuchsia-Auto-Submit: Alex Konradi <akonradi@google.com>
Reviewed-by: Evan Wildenhain <wildenhain@google.com>