Clarify planner_hook calling convention

Hi,

planner hook is frequently used in monitoring and advising extensions.
The call to this hook is implemented in the way, that the
standard_planner routine must be called at least once in the hook's call
chain.

But, as I see in [1], it should allow us "... replace the planner
altogether".
In such situation it haven't sense to call standard_planner at all.
Moreover, if an extension make some expensive planning activity,
monitoring tools, like pg_stat_statements, can produce different
results, depending on a hook calling order.
I thought about additional hooks, explicit hook priorities and so on.
But, maybe more simple solution is to describe requirements to such kind
of extensions in the code and documentation (See patch in attachment)?
It would allow an extension developer legally check and log a situation,
when the extension doesn't last in the call chain.

[1]
https://www.postgresql.org/message-id/flat/27516.1180053940%40sss.pgh.pa.us

--
regards,
Andrey Lepikhov
Postgres Professional