man: remove the {br} macro and reformat the SYNOPSIS

AsciiDoctor sadly seems less flexible than the regular AsciiDoc
program, it cannot handle this macro.  Additionally, reformat the
SYNOPSIS line to the standard AsciiDoc manpage formatting.
man: remove the ":doctype: manpage" directive, disliked by GitHub

This caused the first one or two sections of the manpage rendered on
GitHub to totally disappear.  The Makefile's *.1 and *.html targets
define the manpage doctype explicitly via command-line anyway, so it
will cause no effect using those.
Merge remote-tracking branch 'chungy/master'

From chungy:
The software used on GitHub's side does not support AsciiDoc fully enough to render this properly, but the fixes to work around this should not be too much of a hassle.

* chungy/master:
  man: remove the ":doctype: manpage" directive, disliked by GitHub
  man: remove the {br} macro and reformat the SYNOPSIS