Help: Document CTest Git fetch-and-reset behavior

Help/manual/ctest.1.rst

@@ -589,6 +589,11 @@ Configuration settings to specify the version control tool include:
   * `CTest Script`_ variable: :variable:`CTEST_GIT_COMMAND`
   * :module:`CTest` module variable: ``GITCOMMAND``
 
+  The source tree is updated by ``git fetch`` followed by
+  ``git reset --hard`` to the ``FETCH_HEAD``.  The result is the same
+  as ``git pull`` except that any local moficiations are overwritten.
+  Use ``GITUpdateCustom`` to specify a different approach.
+
 ``GITInitSubmodules``
   If set, CTest will update the repository's submodules before updating.