bpf: Minor fixes to BPF helpers documentation

Minor improvements to the documentation for BPF helpers:

* Fix formatting for the description of "bpf_socket" for
  bpf_getsockopt() and bpf_setsockopt(), thus suppressing two warnings
  from rst2man about "Unexpected indentation".
* Fix formatting for return values for bpf_sk_assign() and seq_file
  helpers.
* Fix and harmonise formatting, in particular for function/struct names.
* Remove blank lines before "Return:" sections.
* Replace tabs found in the middle of text lines.
* Fix typos.
* Add a note to the footer (in Python script) about "bpftool feature
  probe", including for listing features available to unprivileged
  users, and add a reference to bpftool man page.

Thanks to Florian for reporting two typos (duplicated words).

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200511161536.29853-4-quentin@isovalent.com