bpf: add sockopt documentation

Provide user documentation about sockopt prog type and cgroup hooks.

v9:
* add details about setsockopt context and inheritance

v7:
* add description for retval=0 and optlen=-1

v6:
* describe cgroup chaining, add example

v2:
* use return code 2 for kernel bypass

Cc: Andrii Nakryiko <andriin@fb.com>
Cc: Martin Lau <kafai@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>