Help: Add release notes for topic 'FeatureSummary_combine_WHAT_values'

Help/release/dev/FeatureSummary_combine_WHAT_values.rst

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+FeatureSummary_combine_WHAT_values
+----------------------------------
+
+* The :module:`FeatureSummary` module ``feature_summary`` API
+  learned to accept multiple values for the ``WHAT`` option and
+  combine them appropriately.