Help: Add notes for topic 'ExternalProject_init_selected_submodules'

Help/release/dev/ExternalProject_init_selected_submodules.rst

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+ExternalProject_init_selected_submodules
+----------------------------------------
+
+* The :module:`ExternalProject` module :command:`ExternalProject_Add`
+  function ``GIT_SUBMODULES`` option now also limits the set of
+  submodules that are initialized in addition to the prior behavior
+  of limiting the set of submodules that are updated.