[docs] Pattern replace char filter now support flags

docs/reference/analysis/charfilters/pattern-replace-charfilter.asciidoc

@@ -21,6 +21,11 @@ The `pattern_replace` character filter accepts the following parameters:
     `$1`..`$9` syntax, as explained
     http://docs.oracle.com/javase/8/docs/api/java/util/regex/Matcher.html#appendReplacement-java.lang.StringBuffer-java.lang.String-[here].
 
+`flags`::
+
+    Java regular expression http://docs.oracle.com/javase/8/docs/api/java/util/regex/Pattern.html#field.summary[flags].
+    lags should be pipe-separated, eg `"CASE_INSENSITIVE|COMMENTS"`.
+
 [float]
 === Example configuration