Add a documentation about fprobe event tracing including
tracepoint probe event and BTF argument.

Link: https://lore.kernel.org/all/168507479345.913472.2804569685436422001.stgit@mhiramat.roam.corp.google.com/Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>