Update documentation about SIGKILL of postmaster.

Update documentation about SIGKILL of postmaster.

Removing shared memory and semaphores in response to server start
failure often masks the real problem, a live process associated with the
data directory; see commit 5a907404b52753c4d6c6a7c21765aeaa42fd6a3b.
Since 9.6, it's rarely necessary to kill subprocesses manually.  (When
it is necessary, that commit's HINT will say as much, in all supported
versions.)