manual: clarify SNI preference order

docs/MANUAL.md

@@ -622,6 +622,8 @@ Oragono supports [SNI](https://en.wikipedia.org/wiki/Server_Name_Indication); th
                     key:  key2.pem
 ```
 
+If multiple certificates are applicable, or the client does not send SNI, the server will offer the first applicable certificate in the list.
+
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