Documentation: tracing/probes: Add fprobe event tracing document

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>

Documentation/trace/index.rst

@@ -13,6 +13,7 @@ Linux Tracing Technologies
    kprobes
    kprobetrace
    uprobetracer
+   fprobetrace
    tracepoints
    events
    events-kmem

Documentation/trace/kprobetrace.rst

@@ -66,6 +66,8 @@ Synopsis of kprobe_events
   (\*3) this is useful for fetching a field of data structures.
   (\*4) "u" means user-space dereference. See :ref:`user_mem_access`.
 
+.. _kprobetrace_types:
+
 Types
 -----
 Several types are supported for fetchargs. Kprobe tracer will access memory