bpf: add documentation for eBPF helpers (51-57)

Add documentation for eBPF helper functions to bpf.h user header file.
This documentation can be parsed with the Python script provided in
another commit of the patch series, in order to provide a RST document
that can later be converted into a man page.

The objective is to make the documentation easily understandable and
accessible to all eBPF developers, including beginners.

This patch contains descriptions for the following helper functions:

Helpers from Lawrence:
- bpf_setsockopt()
- bpf_getsockopt()
- bpf_sock_ops_cb_flags_set()

Helpers from Yonghong:
- bpf_perf_event_read_value()
- bpf_perf_prog_read_value()

Helper from Josef:
- bpf_override_return()

Helper from Andrey:
- bpf_bind()

v4:
- bpf_perf_event_read_value(): State that this helper should be
  preferred over bpf_perf_event_read().

v3:
- bpf_perf_event_read_value(): Fix time of selection for perf event type
  in description. Remove occurences of "cores" to avoid confusion with
  "CPU".
- bpf_bind(): Remove last paragraph of description, which was off topic.

Cc: Lawrence Brakmo <brakmo@fb.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: Josef Bacik <jbacik@fb.com>
Cc: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Yonghong Song <yhs@fb.com>
[for bpf_perf_event_read_value(), bpf_perf_prog_read_value()]
Acked-by: Andrey Ignatov <rdna@fb.com>
[for bpf_bind()]
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>