Help: Add notes for topic 'FindFLEX-DEFINES_FILE'

Help/release/dev/FindFLEX-DEFINES_FILE.rst

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+FindFLEX-DEFINES_FILE
+---------------------
+
+* The :module:`FindFLEX` module ``FLEX_TARGET`` macro learned a
+  new ``DEFINES_FILE`` option to specify a custom output header
+  to be generated.