Help: Add 3.20 release note section for 3.20.{3,4,5}

Make it clear that the notes for these patch releases were not
forgotten, and that there were simply no changes to documented features
or interfaces.

Help/release/3.20.rst

@@ -362,3 +362,10 @@ Changes made since CMake 3.20.0 include the following.
   a special case to recognize oneAPI 2021.1 Fortran as ``IntelLLVM``.
   The oneAPI 2021.2 Fortran compiler defines the proper identification
   macro and so is identified as ``IntelLLVM`` by all CMake 3.20 versions.
+
+3.20.3, 3.20.4, 3.20.5
+----------------------
+
+These versions made no changes to documented features or interfaces.
+Some implementation updates were made to support ecosystem changes
+and/or fix regressions.