[SPARK-16324][SQL] regexp_extract should doc that it returns empty string when match fails

## What changes were proposed in this pull request?

Doc that regexp_extract returns empty string when regex or group does not match

## How was this patch tested?

Jenkins test, with a few new test cases

Author: Sean Owen <sowen@cloudera.com>

Closes #14525 from srowen/SPARK-16324.

python/pyspark/sql/functions.py

@@ -1440,11 +1440,15 @@ def split(str, pattern):
 @ignore_unicode_prefix
 @since(1.5)
 def regexp_extract(str, pattern, idx):
-    """Extract a specific(idx) group identified by a java regex, from the specified string column.
+    """Extract a specific group matched 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.
 
     >>> df = spark.createDataFrame([('100-200',)], ['str'])
     >>> df.select(regexp_extract('str', '(\d+)-(\d+)', 1).alias('d')).collect()
     [Row(d=u'100')]
+    >>> df = spark.createDataFrame([('foo',)], ['str'])
+    >>> df.select(regexp_extract('str', '(\d+)', 1).alias('d')).collect()
+    [Row(d=u'')]
     >>> df = spark.createDataFrame([('aaaac',)], ['str'])
     >>> df.select(regexp_extract('str', '(a+)(b)?(c)', 2).alias('d')).collect()
     [Row(d=u'')]

sql/core/src/main/scala/org/apache/spark/sql/functions.scala

@@ -2175,7 +2175,8 @@ object functions {
   def ltrim(e: Column): Column = withExpr {StringTrimLeft(e.expr) }
 
   /**
-   * Extract a specific(idx) group identified by a java regex, from the specified string column.
+   * Extract a specific group matched 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.
    *
    * @group string_funcs
    * @since 1.5.0

sql/core/src/test/scala/org/apache/spark/sql/StringFunctionsSuite.scala

@@ -96,6 +96,10 @@ class StringFunctionsSuite extends QueryTest with SharedSQLContext {
 
   test("non-matching optional group") {
     val df = Seq(Tuple1("aaaac")).toDF("s")
+    checkAnswer(
+      df.select(regexp_extract($"s", "(foo)", 1)),
+      Row("")
+    )
     checkAnswer(
       df.select(regexp_extract($"s", "(a+)(b)?(c)", 2)),
       Row("")