Manual:$wgRightsText

Details
If either $wgRightsUrl or $wgRightsPage is specified then this variable gives the text for the link. If using $wgRightsUrl then this value must be specified. It's set at install time by the installer, either from its own predefined list or from the data returned from the Creative Commons license chooser.

If using $wgRightsPage then the name of the page will also be used as the link if this variable is not set.

Example
This will render in the footer as:

Content is available under our special license.

The encapsulating text can be changed in MediaWiki:Copyright.