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 (message added on 2004-06-08). This message is shown in the footer of the page and is not really supposed to contain valuable info which can't be retrieved elsewhere; it's not easily retrievable via API, so it's a bad idea to put here important info especially if you declare to be using a license.