Add missing release snippet for option() deprecated kwarg

docs/markdown/snippets/deprecated_option.md

@@ -0,0 +1,23 @@
+## Deprecated project options
+
+Project options declared in `meson_options.txt` can now be marked as deprecated
+and Meson will warn when user sets a value to it. It is also possible to deprecate
+only some of the choices, and map deprecated values to a new value.
+
+```meson
+# Option fully deprecated, it warns when any value is set.
+option('o1', type: 'boolean', deprecated: true)
+
+# One of the choices is deprecated, it warns only when 'a' is in the list of values.
+option('o2', type: 'array', choices: ['a', 'b'], deprecated: ['a'])
+
+# One of the choices is deprecated, it warns only when 'a' is in the list of values
+# and replace it by 'c'.
+option('o3', type: 'array', choices: ['a', 'b', 'c'], deprecated: {'a': 'c'})
+
+# A boolean option has been replaced by a feature, old true/false values are remapped.
+option('o4', type: 'feature', deprecated: {'true': 'enabled', 'false': 'disabled'})
+
+# A feature option has been replaced by a boolean, enabled/disabled/auto values are remapped.
+option('o5', type: 'boolean', deprecated: {'enabled': 'true', 'disabled': 'false', 'auto': 'false'})
+```