Document repl 'host' setting

The [repl] block also supports setting 'host' to change where the repl service
is listening to. This documents this setting, while also warning that changing
it from 127.0.0.1 is not recommended for a production environment.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #824 from kbarber/maint/1.5.x/repl-host-documentation

DOCS: Document repl 'host' setting