docs: Add release notes for new gnome.gdbus_codegen keywords

docs/markdown/snippets/gdbus_codegen_options.md

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+## New options to gnome.gdbus_codegen
+
+You can now pass additional arguments to gdbus-codegen using the `extra_args`
+keyword. This is the same for the other gnome function calls.
+
+Meson now automatically adds autocleanup support to the generated code. This
+can be modified by setting the autocleanup keyword.
+
+For example:
+
+    sources += gnome.gdbus_codegen('com.mesonbuild.Test',
+      'com.mesonbuild.Test.xml',
+      autocleanup : 'none',
+      extra_args : ['--pragma-once'])