add synopses for MediaWiki-Vagrant commands

This commit adds synopses for MediaWiki-Vagrant's custom commands, so that they
have meaningful help messages when somebody runs `vagrant -h`, hopefully making
it clearer as to what each command does at a glance.

Before this commit, most non-vanilla Vagrant commands had a blank space where
their synopsis would be. `vagrant vbguest` is now the only command missing a
synopsis.

Change-Id: I7173a27cf2f70f44d3387eb1a004874e6b569fa3
Merge "add synopses for MediaWiki-Vagrant commands"