Adding hints that the next_node name should contain a dot to avoid (mobile) browsers interpreting "nextnode" as searchphrase. Also setting it to an fqdn helps when clients use static dns servers.

gluon-core: dns-cache documentation polish
Now its clear that upstreams severs normally means "community gateways".
* added cacheentries to description
* made upstream server description more general
* description of the A and AAAA-records for next_node added
* mention of Cache not being allocated when not set in site.conf 
* dns-cache doku polish