The code of Battle For Wesnoth is issued under the GNU General Public License (GPL), version 2 or later. By contributing code or content to the project, you agree for it to be distributed under GPL or whatever other open-source license the project maintainers choose in the future.
The content of the Wesnoth wiki is under the GNU Free Documentation License (GFDL). By contributing content to the wiki, you agree for the code to be distributed under GFDL or whatever other open-source documentation or content license the project maintainers choose in the future.
(The future-choice qualifier is so we have a way to recover should technical flaws in GPL or GFDL leave us legally unable to distribute the code under those licenses. By "open-source license" we mean one complying with the Open Source Definition.)
As the project owners, with the right to issue copies of the code or content under dual or multiple licenses, the Wesnoth maintainers specifically grant the right to copy content from game to wiki or wiki to game, said copy acquiring only the license obligations associated with the destination.