Help: Add notes for topic 'add-CheckFortranCompilerFlag'

Help/release/dev/add-CheckFortranCompilerFlag.rst

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+add-CheckFortranCompilerFlag
+----------------------------
+
+* The :module:`CheckFortranCompilerFlag` module was introduced
+  to check ``Fortran`` compiler flags, much like the
+  :module:`CheckCCompilerFlag` module already does for ``C``.