`db-channel-enabled` should be reloadable by SIGUSR2

[steve-chavez]

Problem

The docs say that db-channel-enabled is reloadable:

Reloadable: Y
In-Database: n/a

But this is not true.

Previously discussed on #4860 (comment)

[taimoorzaeem]

Right. For the record, the docs also say that db-channel is reloadable, and that's not true either.