Forking a GitHub project does not fork its wiki repo. A blank wiki is created instead in the fork, and any changes to it cannot be merged using pull requests. A workaround to this is to clone the GitHub wiki locally then push it into a separate repository, or a separate repository's wiki, e.g.:
git clone https://github.com/user1/project.wiki.git
git remote add my-fork https://github.com/user2/project.wiki.git
git push my-fork master
To keep the wikis in sync:
git pull origin master
git push my-fork master