doc: media: visl: Document tpg_verbose parameter

Also document stable frames and what it means for testing tools.

Signed-off-by: Detlev Casanova <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>
[hverkuil: add media: prefix to Subject]

Documentation/admin-guide/media/visl.rst

···
   visl_dprintk_frame_start, visl_dprintk_nframes, but controls the dumping of
   buffer data through debugfs instead.
 
+- tpg_verbose: Write extra information on each output frame to ease debugging
+  the API. When set to true, the output frames are not stable for a given input
+  as some information like pointers or queue status will be added to them.
+
 What is the default use case for this driver?
 ---------------------------------------------
 
···
 OUTPUT buffer data is subsequently used to debug a work-in-progress
 implementation.
 
-Information on reference frames, their timestamps, the status of the OUTPUT and
-CAPTURE queues and more can be read directly from the CAPTURE buffers.
+Even though no video decoding is actually done, the output frames can be used
+against a reference for a given input, except if tpg_verbose is set to true.
+
+Depending on the tpg_verbose parameter value, information on reference frames,
+their timestamps, the status of the OUTPUT and CAPTURE queues and more can be
+read directly from the CAPTURE buffers.
 
 Supported codecs
 ----------------