tools: bpftool: add doc for -m option to bpftool-prog.rst

The --mapcompat|-m option has been documented on the main bpftool.rst
page, and on the interactive help. As this option is useful for loading
programs with maps with the "bpftool prog load" command, it should also
appear in the related bpftool-prog.rst documentation page. Let's add it.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>