warn of possible client-side SSL requirement

MacOS's Calendar.app may not send auth credentials when connecting to a CalDAV
server over unsecured HTTP. Attempting to set up an account that way will give
error messages about authentication, even though the credential the user
entered are correct.

Here, we point out this case in the documentation, prompting users to try
enabling SSL if they encounter this.

DOCUMENTATION.md

@@ -1171,6 +1171,11 @@ In some clients you can just enter the URL of the Radicale server
 enter the URL of the collection directly
 (e.g. `http://localhost:5232/user/calendar`).
 
+Some clients (notably macOS's Calendar.app) may silently refuse to include
+account credentials over unsecured HTTP, leading to unexpected authentication
+failures. In these cases, you want to make sure the Radicale server is
+[accessible over HTTPS](#ssl).
+
 #### DAVx⁵
 
 Enter the URL of the Radicale server (e.g. `http://localhost:5232`) and your