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

.env.production.sample

@@ -14,7 +14,8 @@ LOCAL_DOMAIN=example.com
 LOCAL_HTTPS=true
 
 # Use this only if you need to run mastodon on a different domain than the one used for federation.
-# Do not use this unless you know exactly what you are doing.
+# You can read more about this option on https://github.com/tootsuite/documentation/blob/master/Running-Mastodon/Serving_a_different_domain.md
+# DO *NOT* USE THIS UNLESS YOU KNOW *EXACTLY* WHAT YOU ARE DOING.
 # WEB_DOMAIN=mastodon.example.com
 
 # Application secrets