[hotfix] [table] Remove wrong ISO references for timestamp extraction

The docs and timestamp extractors mention ISO timestamps, however, according
to ISO 8601 timestamps have a different format.

docs/dev/table/sourceSinks.md

@@ -460,8 +460,7 @@ val source: KafkaTableSource = Kafka010JsonTableSource.builder()
 Flink provides `TimestampExtractor` implementations for common use cases.
 The following `TimestampExtractor` implementations are currently available:
 
-* `ExistingField(fieldName)`: Extracts the value of a rowtime attribute from an existing `LONG` or `SQL_TIMESTAMP`, or ISO date formatted `STRING` field.
-  * One example of ISO date format would be '2018-05-28 12:34:56.000'.
+* `ExistingField(fieldName)`: Extracts the value of a rowtime attribute from an existing `LONG`, `SQL_TIMESTAMP`, or timestamp formatted `STRING` field. One example of such a string would be '2018-05-28 12:34:56.000'.
 * `StreamRecordTimestamp()`: Extracts the value of a rowtime attribute from the timestamp of the `DataStream` `StreamRecord`. Note, this `TimestampExtractor` is not available for batch table sources.
 
 A custom `TimestampExtractor` can be defined by implementing the corresponding interface.

flink-libraries/flink-table/src/main/scala/org/apache/flink/table/sources/tsextractors/ExistingField.scala

@@ -23,8 +23,9 @@ import org.apache.flink.table.api.{Types, ValidationException}
 import org.apache.flink.table.expressions.{Cast, Expression, ResolvedFieldReference}
 
 /**
-  * Converts an existing [[Long]] or [[java.sql.Timestamp]], or
-  * ISO date formatted [[java.lang.String]] field into a rowtime attribute.
+  * Converts an existing [[Long]], [[java.sql.Timestamp]], or
+  * timestamp formatted [[java.lang.String]] field (e.g., "2018-05-28 12:34:56.000") into
+  * a rowtime attribute.
   *
   * @param field The field to convert into a rowtime attribute.
   */
@@ -47,8 +48,9 @@ final class ExistingField(val field: String) extends TimestampExtractor {
   }
 
   /**
-    * Returns an [[Expression]] that casts a [[Long]] or [[java.sql.Timestamp]], or
-    * ISO date formatted [[java.lang.String]] field into a rowtime attribute.
+    * Returns an [[Expression]] that casts a [[Long]], [[java.sql.Timestamp]], or
+    * timestamp formatted [[java.lang.String]] field (e.g., "2018-05-28 12:34:56.000")
+    * into a rowtime attribute.
     */
   override def getExpression(fieldAccesses: Array[ResolvedFieldReference]): Expression = {
     val fieldAccess: Expression = fieldAccesses(0)