Describe the behavior for specifying invalid mode

Author: AnthonyLatsis

proposals/NNNN-adoption-tooling-for-swift-features.md

@@ -148,6 +148,9 @@ For example:
 -enable-upcoming-feature InternalImportsByDefault:adoption
 ```
 
+If the specified mode is invalid, the flag will be ignored, and a warning will
+be emitted. This warning will belong to the `StrictLanguageFeatures` diagnostic
+group.
 In a series of either of these options applied to a given feature, only the
 last option will be honored.
 If an upcoming feature is both implied by the effective language mode and