[SPARK-4437] update doc for WholeCombineFileRecordReader

update doc for WholeCombineFileRecordReader

Author: Davies Liu <davies@databricks.com>
Author: Josh Rosen <joshrosen@databricks.com>

Closes #3301 from davies/fix_doc and squashes the following commits:

1d7422f [Davies Liu] Merge pull request #2 from JoshRosen/whole-text-file-cleanup
dc3d21a [Josh Rosen] More genericization in ConfigurableCombineFileRecordReader.
95d13eb [Davies Liu] address comment
bf800b9 [Davies Liu] update doc for WholeCombineFileRecordReader