doc: Mention UNION/ORDER BY etc. keywords in section headers.

doc: Mention UNION/ORDER BY etc. keywords in section headers.

Most of the section and sub-section headers in the Queries chapter have
the keywords literally stated, but neither "Sorting Rows" nor "Combining
Rows" did. There's no rule that they must be, but it seems like a good
practice. The keywords will ring a bell to anyone with with even a little
bit of SQL experience.

David G. Johnston, per suggestion by bilge@scriptfusion.com

Discussion: https://www.postgresql.org/message-id/159981394174.31338.7014519396749859167%40wrigleys.postgresql.org