tracing: Add better comments for the filtering temp buffer use case

When filtering is enabled, the event is copied into a temp buffer instead
of being written into the ring buffer directly, because the discarding of
events from the ring buffer is very expensive, and doing the extra copy is
much faster than having to discard most of the time.

As that logic is subtle, add comments to explain in more detail to what is
going on and how it works.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>