gh-89554: Document _thread.LockType as a class (#150684)

gh-89554: Document _thread.LockType as a class (#150684)

_thread.LockType is a class (the type of lock objects), but was documented
with the ".. data::" directive, so ":class:" cross-references to it cannot
resolve against a py:class target.

Switch the entry to ".. class::", move it next to the lock methods, and
document acquire(), release() and locked() as methods of the class.  Keep
the old _thread.lock.* URL fragments working with raw HTML anchors.