add qldoc to RegExpCharEscape

ruby/ql/lib/codeql/ruby/regexp/RegExpTreeView.qll

@@ -539,6 +539,15 @@ private module Impl implements RegexTreeViewSig {
     override predicate isNullable() { this.getAChild().isNullable() }
   }
 
+  /**
+   * A character escape in a regular expression.
+   *
+   * Example:
+   *
+   * ```
+   * \.
+   * ```
+   */
   class RegExpCharEscape = RegExpEscape;
 
   /**