docs(guide/expression): add clarification for RegExp literal in ngPattern expression

The `ngPattern` expression does accept a RegExp created with literal notation,
hence it should be mentioned as an exception to the
"No RegExp Creation With Literal Notation" rule.

Closes #16206

Author: YonatanKra

docs/content/guide/expression.ngdoc

@@ -37,7 +37,8 @@ AngularJS expressions are like JavaScript expressions with the following differe
     even inside `ng-init` directive.
 
   * **No RegExp Creation With Literal Notation:** You cannot create regular expressions
-    in an AngularJS expression.
+    in an AngularJS expression. An exception to this rule is {@link ngPattern `ng-pattern`} which accepts valid
+    RegExp.
 
   * **No Object Creation With New Operator:** You cannot use `new` operator in an AngularJS expression.