xsk: Improve documentation for AF_XDP

Added sections on all the bind flags, libbpf, all the setsockopts and
all the getsockopts. Also updated the document to reflect the latest
features and to correct some spelling errors.

v1 -> v2:
* Updated XDP program with latest BTF map format
* Added one more FAQ entry
* Some minor edits and corrections

v2 -> v3:
* Simplified XDP_SHARED_UMEM example XDP program

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/1571648224-16889-1-git-send-email-magnus.karlsson@intel.com