docs/bpf: Add documentation for BPF_MAP_TYPE_SK_STORAGE

Add documentation for the BPF_MAP_TYPE_SK_STORAGE including
kernel version introduced, usage and examples.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20221209112401.69319-1-donald.hunter@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>