Correct the documentation of the `BLI_uniquename_cb()` variant that was
introduced in 7afa5aaa59 to support
dynamically-allocated strings.
Contrary to the other functions with the same name, this variant does
not actually assign the name. It returns the new name and leaves the
assignment to the caller.
Pull Request: https://projects.blender.org/blender/blender/pulls/139639