BUG #15558: NOTIFY max channel length is undocumented

The following bug has been logged on the website:

Bug reference:      15558
Logged by:          Andy Edwards
Email address:      jedwards@fastmail.com
PostgreSQL version: 11.1
Operating system:   (irrelevant)
Description:

After debugging why notifications weren't getting delivered, I discovered
the following:

# LISTEN "pg/tables/TagNotificationTriggers/tag/_org/1/JSON
Test/battery/1/lifetimeWh";
NOTICE:  identifier "pg/tables/TagNotificationTriggers/tag/_org/1/JSON
Test/battery/1/lifetimeWh" will be truncated to
"pg/tables/TagNotificationTriggers/tag/_org/1/JSON Test/battery/"

I haven't seen a mention of this channel length limit in any version of the
documentation for NOTIFY.  I wish it were in the documentation, because then
I would have found out about this limit before deciding to use Postgres
notifications, and chosen to use Redis notifications from the beginning.