Reduces the recursion count for write mode, and exits write mode if the resulting count is 0 (zero).
This method is not sensitive to recursion order. For example, if a thread enters a lock in upgradeable mode and then enters the lock in write mode, the order in which the thread exits the two modes does not matter. If a lock allows recursion, a thread can enter the lock in write mode and then enter it recursively in read mode; the order in which the thread exits read mode and write mode does not matter.
Exiting the lock might signal other waiting threads.