.gitkeep isn’t documented, because it’s not a feature of Git.
Git cannot add a completely empty directory. People who want to track empty directories in Git have created the convention of putting files called .gitkeep in these directories. The file could be called anything; Git assigns no special significance to this name.
There is a competing convention of adding a .gitignore file to the empty directories to get them tracked, but some people see this as confusing since the goal is to keep the empty directories, not ignore them; .gitignore is also used to list files that should be ignored by Git when looking for untracked files.