modules/priming: don't warn against unloading it

I can't see sufficient motivation here.  The cache will be slightly
less ready, but it's not often that you need to contact a root server.

Most importantly, kresd must work well anyway, even with empty cache.
Also, the compiled-in address set of root servers should be quite
accurate - the NS set has never changed, and the last address change
was five years ago with just one of 26 records changing.

modules/priming/README.rst

@@ -14,6 +14,5 @@ the resolver software. Unlike other DNS resolvers, Knot Resolver caches
 result of priming query on disk and keeps the data between restarts until
 TTL expires.
 
-This module is enabled by default and it is not recommended to disable it.
-For debugging purposes you may disable the module by appending
+This module is enabled by default; you may disable it by adding
 ``modules.unload('priming')`` to your configuration.