tools: bpftool: make syntax for program map update explicit in man page

tools: bpftool: make syntax for program map update explicit in man page

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: ff69c21a85a4 ("tools: bpftool: add documentation")
Signed-off-by: Quentin Monnet <[email protected]>
Reviewed-by: Jakub Kicinski <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>