docs(error): updated description for ngPattern/noregexp.ngdoc

Author: naomiblack

docs/content/error/ngPattern/noregexp.ngdoc

@@ -2,3 +2,6 @@
 @name ngPattern:noregexp
 @fullName Expected Regular Expression
 @description
+This error occurs when 'ngPattern' is passed an expression that isn't a regular expression or doesn't have the expected format.
+
+For more information on valid expression syntax, see 'ngPattern' in {@link api/ng.directive:select input} directive docs.