bind: Expand description of services.bind.cacheNetworks

The new description should give more clear understanding of when to
edit the option.

I used NixOS to set up a DNS server that is authoritative for certain
zones.  The description of the `cacheNetworks` option made me think I
needed to set it to `"any"` to allow people to query the zone I set
up.  Reading the source of the module would have clarified my
understanding, but at the time I just read the description and thought
little of it.  Later I discovered I was getting tons of DNS requests
and presumably being used for a DNS amplification attack or similar.
I have fixed the problem now, but I would like the option to have a
clearer description so others don't make the same mistake I did.