ir: make bt_ctf_trace_create() create an empty packet header FT

Currently bt_ctf_trace_create() creates an initial default packet header
field type with the `magic`, `uuid`, and `stream_id` fields. This is, in
fact, specific to CTF writer. Since the trace API is not public yet,
move this default packet header FT creation from bt_ctf_trace_create()
to bt_ctf_writer_create() to ensure backward compatibility.

This patch also fixes a few things in the API doc. of trace.h.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>