Update u_e-unbound-fwd.md

Add additional details why public revolvers shouldn't be used

docs/u_e-unbound-fwd.md

@@ -1,7 +1,7 @@
 If you want or have to use an external DNS service, you can either set a forwarder in Unbound or copy an override file to define external DNS servers:
 
    !!! warning
-   Please do not use a public resolver like we did in the example above. Many - if not all - blacklist lookups will fail with public resolvers.
+   Please do not use a public resolver like we did in the example above. Many - if not all - blacklist lookups will fail with public resolvers, because blacklist server has limits on how much requests can be done from one IP and public resolvers usually reach this limits.
    **Important**: Only DNSSEC validating DNS services will work.
 
 ## Method A, Unbound