Add #unavailable to the list of keywords.

Co-authored-by: Scott Richmond (GitHub user @tadbyt)

Author: amartini51

TSPL.docc/ReferenceManual/LexicalStructure.md

@@ -324,7 +324,8 @@ so they must be escaped with backticks in that context.
   `#imageLiteral`,
   `#keyPath`,
   `#selector`,
-  `#sourceLocation`.
+  `#sourceLocation`,
+  `#unavailable`.
 
 > Note:
 > Prior to Swift 5.9,