Add additional documentation and warnings to the WEB_DOMAIN setting. (#2386)

Add additional documentation and warnings to the WEB_DOMAIN setting. (#2386)

* Add additional documentation and warnings to the WEB_DOMAIN setting.

This feature is largely undocumented, and quite a number of users have
shot them in the feet already despite the warning. Added a bit of documentation
and expanded the warning until we have a mechanism for dealing with conflicting
user URIs.

* Change WEB_DOMAIN comments to point to the extensive online documentation