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

scripts/bpf_helpers_doc.py

@@ -318,6 +318,11 @@ may be interested in:
   of eBPF maps are used with a given helper function.
 * *kernel/bpf/* directory contains other files in which additional helpers are
   defined (for cgroups, sockmaps, etc.).
+* The bpftool utility can be used to probe the availability of helper functions
+  on the system (as well as supported program and map types, and a number of
+  other parameters). To do so, run **bpftool feature probe** (see
+  **bpftool-feature**\ (8) for details). Add the **unprivileged** keyword to
+  list features available to unprivileged users.
 
 Compatibility between helper functions and program types can generally be found
 in the files where helper functions are defined. Look for the **struct
@@ -338,6 +343,7 @@ SEE ALSO
 ========
 
 **bpf**\ (2),
+**bpftool**\ (8),
 **cgroups**\ (7),
 **ip**\ (8),
 **perf_event_open**\ (2),