Manpage: Update information regarding global filters and multiplexer.

Update the manual page of avocado to match the current status of the multiplexer,
regarding command line options for global filters.

Fix information on command line option -c | --content. It's about variables.

Plus:  cosmetic fixes around.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
Merge pull request #233 from ruda/manpage_multiplex_filters

Manpage: Update information regarding global filters and multiplexer.