docs/autoupdater: depends on ipv6 reachable update-servers

docs/features/autoupdater.rst

@@ -54,7 +54,7 @@ We suggest to have following directory tree accessible via http:
                     sysupgrade/
                     factory/
 
-The server should be available via IPv6.
+The server must be available via IPv6.
 
 Command Line
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