Chapter 7. Queries

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/12/queries.html
Description:

I like to browse documentation by index rather than searching. The Postgres
documentation is fairly well organized so this is easy for most topics.
However, on the "Chapter 7. Queries" index page, I often think I am in the
wrong place if I am looking for information about UNION or ORDER BY, because
those keywords are missing from that page. Unlike all the other headings,
the headings for these two topics does not include the relevant keywords in
teletype. I think it is important to include the keywords, particularly for
people whom already know which keyword they're looking for.

For example, I propose the following changes:
7.4. Combining Queries -> 7.4. Combining Queries with UNION
7.5. Sorting Rows -> 7.5. Sorting Rows with ORDER BY

If there are other examples I've missed where a topic is bound to a specific
keyword, they should updated similarly.