[SPARK-6994][SQL] Update docs for fetching Row fields by name
[SPARK-6994][SQL] Update docs for fetching Row fields by name

add docs for https://issues.apache.org/jira/browse/SPARK-6994

Author: vidmantas zemleris <vidmantas@vinted.com>

Closes #6030 from vidma/docs/row-with-named-fields and squashes the following commits:

241b401 [vidmantas zemleris] [SPARK-6994][SQL] Update docs for fetching Row fields by name

(cherry picked from commit 640f63b959f936ac142ce429262ffc3db4536619)
Signed-off-by: Reynold Xin <c9d35a384bc579d1c6c8c055f2a2b5c04e11b521@databricks.com>

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