Autogen: Add AUTOGEN_SOURCE_GROUP release notes

Help/release/dev/Autogen_source_group.rst

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+AutoGen source group
+--------------------
+
+* Global properties :prop_gbl:`AUTOGEN_SOURCE_GROUP`,
+  :prop_gbl:`AUTOMOC_SOURCE_GROUP` and
+  :prop_gbl:`AUTORCC_SOURCE_GROUP` were
+  introduced to allow files generated by :prop_tgt:`AUTOMOC` or
+  :prop_tgt:`AUTORCC` to be placed in a :command:`source_group`.