doc: document -opencl_options option in ff* tools manuals

doc/avtools-common-opts.texi

@@ -240,6 +240,13 @@ Possible flags for this option are:
 @end table
 @end table
 
+@item -opencl_options options (@emph{global})
+Set OpenCL environment options. This option is only available when
+FFmpeg has been compiled with @code{--enable-opencl}.
+
+@var{options} must be a list of @var{key}=@var{value} option pairs
+separated by ':'. See the ``OpenCL Options'' section in the
+ffmpeg-utils manual for the list of supported options.
 @end table
 
 @section AVOptions