Move un-parenthesized syntax docs to "compatibility" for few SQL commands

Hi,

I find the inclusion of the un-parenthesized syntax for VACUUM, ANALYZE,
and EXPLAIN in the docs makes it harder to understand the preferred,
parenthesized syntax (see [1] as an example).

Over in [2], it was suggested that moving the un-parenthesized syntax to
the "Compatibility" section of their respective docs pages would be
okay. Patch attached.

I left out CLUSTER since that syntax change is so new.

- Melanie

[1] https://www.postgresql.org/docs/devel/sql-analyze.html
[2] https://www.postgresql.org/message-id/3024596.1681940741%40sss.pgh.pa.us