bpf: Document BPF_MAP_*_BATCH syscall commands

Based roughly on the following commits:
* Commit cb4d03ab ("bpf: Add generic support for lookup batch op")
* Commit 05799638 ("bpf: Add batch ops to all htab bpf map")
* Commit aa2e93b8 ("bpf: Add generic support for update and delete
  batch ops")

Signed-off-by: Joe Stringer <joe@cilium.io>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Brian Vazquez <brianvv@google.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210302171947.2268128-9-joe@cilium.io