[UR] Add lifetime validation documentation

scripts/core/INTRO.rst

@@ -256,6 +256,9 @@ Layers currently included with the runtime are as follows:
      - Enables non-adapter-specific parameter validation (e.g. checking for null values).
    * - UR_LAYER_LEAK_CHECKING
      - Performs some leak checking for API calls involving object creation/destruction.
+   * - UR_LAYER_LIFETIME_VALIDATION
+     - Performs lifetime validation on objects used in API calls.
+     - Requires UR_LAYER_LEAK_CHECKING.
    * - UR_LAYER_FULL_VALIDATION
      - Enables UR_LAYER_PARAMETER_VALIDATION and UR_LAYER_LEAK_CHECKING.
    * - UR_LAYER_TRACING