tools: bpftool: add a note on program statistics in man page

Linux kernel now supports statistics for BPF programs, and bpftool is
able to dump them. However, these statistics are not enabled by default,
and administrators may not know how to access them.

Add a paragraph in bpftool documentation, under the description of the
"bpftool prog show" command, to explain that such statistics are
available and that their collection is controlled via a dedicated sysctl
knob.

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