bpf: Document BPF_PROG_PIN syscall command

Commit b2197755 ("bpf: add support for persistent maps/progs")
contains the original implementation and git logs, used as reference for
this documentation.

Also pull in the filename restriction as documented in commit 6d8cb045
("bpf: comment why dots in filenames under BPF virtual FS are not allowed")

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: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210302171947.2268128-5-joe@cilium.io