avocado.docs: Update documentation regarding argument parsing.

Update documentation regarding the last update on argument parsing.
Like option --silent goes to run subcommand and there's no
need to quote for multiples test names.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #170 from ruda/argument_parse_docs

avocado.docs: Update documentation regarding argument parsing.