JS: improve documentation on deprecated CryptographicOperation#getInput() predicate

Author: alexrford

javascript/ql/lib/semmle/javascript/Concepts.qll

@@ -118,7 +118,13 @@ module Cryptography {
   private import semmle.javascript.internal.ConceptsShared::Cryptography as SC
 
   class CryptographicOperation extends SC::CryptographicOperation instanceof CryptographicOperation::Range {
-    /** DEPRECATED. This predicate has been renamed to `getAnInput`. */
+    /**
+     * DEPRECATED. This predicate has been renamed to `getAnInput`.
+     *
+     * To implement `CryptographicOperation`, please extend
+     * `CryptographicOperation::Range` and implement `getAnInput` instead of
+     * extending this class directly.
+     */
     deprecated final DataFlow::Node getInput() { result = this.getAnInput() }
   }