BUG #15479: Documentation claims that client_min_messages is relatedto logging

The following bug has been logged on the website:

Bug reference:      15479
Logged by:          Clemens Ladisch
Email address:      clemens@ladisch.de
PostgreSQL version: 11.0
Operating system:   any
Description:

Step 1: User wants to reduce the amount of logging, looks into section
19.8.2 "When To Log".
Step 2: Sets "client_min_messages = fatal".
Step 3: The server sends no ErrorResponse, so erroneous SQL statements
appear to succeed.

Example:
<https://www.postgresql.org/message-id/flat/EE586BE92A4AFB45B03310C2A0C0565D6D0EFC17%40G01JPEXMBKW03>

The problem is that client_min_messages has *no relation whatsoever* with
the server's logging.
This variable affects the client connection, so its documentation should be
moved into section 19.11 "Client Connection Behaviour".
(And the word "messages" might still be misleading; maybe use "network
protocol messages".)