Such information can easly be found on the
Official Homepage and it's
Help Section. In this case the
Command List is what you need.
Another way to find such things out is explore the command list in Messenger Plus! itself. To do this you need to type
/ in a conversation and a small toolwindow will popup with all the available commands, when selecting a command a tooltip will be shown which explains the command.
In case this popup doesn't appear, you need to enable it:
Plus! > Preferences > Instant Messages > Commands & Tags > Enable Command Helper windows