tracing/user_events: Update ABI documentation to align to bits vs bytes

Update the documentation to reflect the new ABI requirements and how to
use the byte index with the mask properly to check event status.

Link: https://lkml.kernel.org/r/20220728233309.1896-7-beaub@linux.microsoft.com

Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>