Ruby: add missing qldoc

Author: alexrford

ruby/ql/lib/codeql/ruby/Concepts.qll

@@ -817,6 +817,14 @@ module Logging {
   }
 }
 
+/**
+ * Provides models for cryptographic concepts.
+ *
+ * Note: The `CryptographicAlgorithm` class currently doesn't take weak keys into
+ * consideration for the `isWeak` member predicate. So RSA is always considered
+ * secure, although using a low number of bits will actually make it insecure. We plan
+ * to improve our libraries in the future to more precisely capture this aspect.
+ */
 module Cryptography {
   import codeql.ruby.internal.ConceptsShared::Cryptography
 }