Minor comment edits in nodeGather.c

While going through nodeGather.c, I noticed portions of the file header
comment that may have been obsoleted by recent revisions of the relevant
parellelism code. For example, there is a reference to PartialSeqScan node
which did not make it into the tree. Attached fixes it. Also, wondering if
the semantics of Gather node is that of Scan or more generic Plan? That is
to ask whether the following edit makes sense:

* nodeGather.c
- * Support routines for scanning a plan via multiple workers.
+ * Support routines for getting the result from a plan via multiple
+ * workers.
*

Thanks,
Amit