Help: Add 3.20 release note for error on unknown arguments

This was left out of commit 1b6c5333 (cmake: Error out on unknown
arguments starting with `-`., 2020-11-30, v3.20.0-rc1~370^2).

Fixes: #22060

Help/manual/cmake.1.rst

@@ -105,6 +105,7 @@ Generator
   is already configured in the shell.  When using one of the
   :ref:`IDE Build Tool Generators`, no particular environment is needed.
 
+.. _`Generate a Project Buildsystem`:
 
 Generate a Project Buildsystem
 ==============================

Help/release/3.20.rst

@@ -298,6 +298,10 @@ Deprecated and Removed Features
 Other Changes
 =============
 
+* When running :manual:`cmake(1)` to :ref:`Generate a Project Buildsystem`,
+  unknown command-line arguments starting with a hyphen (``-``) are now
+  rejected with an error.  Previously they were silently ignored.
+
 * Source file extensions must now be explicit.
   See policy :policy:`CMP0115` for details.