Help: Add notes for topic 'ExternalProject_UPDATE_DISCONNECTED'

Help/release/dev/ExternalProject_UPDATE_DISCONNECTED.rst

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+ExternalProject_UPDATE_DISCONNECTED
+-----------------------------------
+
+* The :module:`ExternalProject` module ``ExternalProject_Add`` command
+  learned an ``UPDATE_DISCONNECTED`` option to avoid automatically
+  updating the source tree checkout from version control.