bpf: sk_lookup: Add user documentation

Describe the purpose of BPF sk_lookup program, how it can be attached, when
it gets invoked, and what information gets passed to it. Point the reader
to examples and further documentation.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200821100226.403844-1-jakub@cloudflare.com