Diagnostics: Add release notes

Help/release/dev/diagnostics.rst

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+Diagnostics
+-----------
+
+* CMake diagnostic actions are now tracked in a new state type managed by the
+  :command:`cmake_diagnostic` command.  This replaces the old system which
+  sometimes used variables to manage reporting actions and sometimes did not
+  allow management beyond command-line options and presets.  The new system
+  also introduces several new diagnostic categories and significantly reduces
+  the cost of adding additional categories in the future.  See the
+  :manual:`cmake-diagnostics(7)` manual for a list of available categories.
+
+Commands
+--------
+
+* The :command:`cmake_diagnostic` command was added to manipulate CMake's
+  diagnostic state.
+
+* The :command:`block` command can now create a ``DIAGNOSTICS`` scope.
+
+* The :command:`include` command now has a ``NO_DIAGNOSTIC_SCOPE`` option.
+
+Deprecated and Removed Features
+-------------------------------
+
+* The ``-W[no-][error=]dev`` command-line options are deprecated.  The new
+  spelling is :option:`-W[no-][error=]author <cmake -W>`.
+
+* The ``--warn-uninitialized`` command-line option is deprecated.  The new
+  spelling is :option:`-Wuninitialized <cmake -W>`.
+
+* The ``--no-warn-unused-cli`` command-line option is deprecated.  The new
+  spelling is :option:`-Wno-unused-cli <cmake -Wno->`.
+
+* The :variable:`CMAKE_WARN_DEPRECATED` and :variable:`CMAKE_ERROR_DEPRECATED`
+  variables are deprecated.  The new :command:`cmake_diagnostic` command should
+  be used instead.