ExternalProject: Extend documentation about GIT_TAG usage

When GIT_SHALLOW is used, the '--depth 1 --no-single-branch'
arguments are add. It means that only branch names and tags
is downloaded to repository. Most Commit Hash is not working.
With this commit the documentation was updated, to describe
the limitation of GIT_SHALLOW.

Modules/ExternalProject.cmake

@@ -251,6 +251,9 @@ External Project Definition
           The lack of such deterministic behavior makes the main project lose
           traceability and repeatability.
 
+        If ``GIT_SHALLOW`` is enabled then ``GIT_TAG`` works only with
+        branch names and tags.  A commit hash is not allowed.
+
       ``GIT_REMOTE_NAME <name>``
         The optional name of the remote. If this option is not specified, it
         defaults to ``origin``.