Update qtaguid tag/untag documentation.

The qtaguid_tagSocket() function tags a network socket by passing a
reference to the given socket to the qtaguid kernel module. The module
will keep the socket alive even if the process calls close() on said
socket. In this scenario, the socket object would not be destroyed
even if all the file descriptor.

While this is at least a memory leak, it plays bad with epoll(7)
if you also didn't remove the socket from the epoll fd before closing
since epoll will not notice that the socket was closed and there is no
way to remove the socket from epoll after it was closed.

This patch updates the documentation to explicitly mention that the
socket must be untag before closing or bad things happen.

Bug: 36264049
Test: None.
Change-Id: I564a9b6d11d22b43a6c12312524386c0338b42ed