tools: bpftool: Log info-level messages when building bpftool man pages

To build man pages for bpftool (and for eBPF helper functions), rst2man
can log different levels of information. Let's make it log all levels
to keep the RST files clean.

Doing so, rst2man complains about double colons, used for literal
blocks, that look like underlines for section titles. Let's add the
necessary blank lines.

v2:
- Use "--verbose" instead of "-r 1" (same behaviour but more readable).
- Pass it through a RST2MAN_OPTS variable so we can easily pass other
  options too.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200909162251.15498-2-quentin@isovalent.com