tools: bpftool: doc, add text about feature-subcommand

This patch adds missing information about feature-subcommand in
bpftool.rst

Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>