Fix man pages to not use unescaped --

I've seen that man pages, as generated by the version of pandoc we
use, show single dashes in option names instead of double ones (-
versus --). After bringing it up with upstream
(http://groups.google.com/group/pandoc-discuss/browse_thread/thread/9c4589a4001d42f9/95ee8dae8932dc93

),
it seems that this is a known behaviour of pandoc that has been
improved in newer versions.

Until then, let's use correctly double dashes; from the two options in
the above thread, I chose to use \-- as that doesn't change the actual
output; whereas ``--nodes`` make this a code block, which will look
differently from a short option and could change how the output looks
(e.g. when in a bold span).

Additionally, I've removed two cases where unescape em was explicitly
intended, as that makes automated checking harder and we can use other
formatting.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>