Specify in the documentation that when using bpftool to update a map of
type BPF_MAP_TYPE_PROG_ARRAY, the syntax for the program used as a value
should use the "id|tag|pinned" keywords convention, as used with
"bpftool prog" commands.

Fixes: ff69c21a ("tools: bpftool: add documentation")
Signed-off-by: NQuentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>