Document damos_before_apply tracepoint on the usage document.

Link: https://lkml.kernel.org/r/20230913022050.2109-3-sj@kernel.orgSigned-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>