docs/bpf: Reword docs for BPF_MAP_TYPE_SK_STORAGE

Improve the grammar of the function descriptions and highlight
that the key is a socket fd.

Fixes: f3212ad5b7e9 ("docs/bpf: Add documentation for BPF_MAP_TYPE_SK_STORAGE")
Reported-by: Martin KaFai Lau <martin.lau@linux.dev>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20221212101600.56026-1-donald.hunter@gmail.com

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>