[SPARKR][DOC] regexp_extract should doc that it returns empty string when match fails

## What changes were proposed in this pull request?

Doc change - see https://issues.apache.org/jira/browse/SPARK-16324

## How was this patch tested?

manual check

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #14934 from felixcheung/regexpextractdoc.

R/pkg/R/functions.R

@@ -2876,7 +2876,8 @@ setMethod("randn", signature(seed = "numeric"),
 
 #' regexp_extract
 #'
-#' Extract a specific(idx) group identified by a java regex, from the specified string column.
+#' Extract a specific \code{idx} group identified by a Java regex, from the specified string column.
+#' If the regex did not match, or the specified group did not match, an empty string is returned.
 #'
 #' @param x a string Column.
 #' @param pattern a regular expression.