[UI] Unclear how to separate DNS servers

# Steps to reproduce

1. In the web ui, go to the subnets page
2. click on a CIDR range.
3. click edit, and edit the DNS field

# expected behavior

Something on that page should tell the user that multiple entries should be separated by a space (and not a comma, semicolon etc). Either a dynamic message that appears when they enter a comma, or a hint in the field itself when empty, or text near that field.

# actual behavior

There is nothing to tell the user whether the field even accepts multiple entries at all.
There is nothing to tell the user that they should not use commas or semicolons to separate multiple entries.

If the user tries to save after using commas, an error message appears. That's great.
However I just tried using a space initially, and I had no idea whether it worked or not. I could not even tell whether multiple fields were allowed. I didn't know whether MAAS checked my input and found it valid, or whether it blindly passed it through without checking for the correct separator.

```
$ dpkg -l '*maas*'|cat
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
+++-===============================-==============================-============-=================================================
ii maas 2.4.2-7034-g2f5deb8b8-0ubuntu1 all "Metal as a Service" is a physical cloud and IPAM
ii maas-cli 2.4.2-7034-g2f5deb8b8-0ubuntu1 all MAAS client and command-line interface
un maas-cluster-controller <none> <none> (no description available)
ii maas-common 2.4.2-7034-g2f5deb8b8-0ubuntu1 all MAAS server common files
ii maas-dhcp 2.4.2-7034-g2f5deb8b8-0ubuntu1 all MAAS DHCP server
un maas-dns <none> <none> (no description available)
ii maas-proxy 2.4.2-7034-g2f5deb8b8-0ubuntu1 all MAAS Caching Proxy
ii maas-rack-controller 2.4.2-7034-g2f5deb8b8-0ubuntu1 all Rack Controller for MAAS
ii maas-region-api 2.4.2-7034-g2f5deb8b8-0ubuntu1 all Region controller API service for MAAS
ii maas-region-controller 2.4.2-7034-g2f5deb8b8-0ubuntu1 all Region Controller for MAAS
un maas-region-controller-min <none> <none> (no description available)
un python-django-maas <none> <none> (no description available)
un python-maas-client <none> <none> (no description available)
un python-maas-provisioningserver <none> <none> (no description available)
ii python3-django-maas 2.4.2-7034-g2f5deb8b8-0ubuntu1 all MAAS server Django web framework (Python 3)
ii python3-maas-client 2.4.2-7034-g2f5deb8b8-0ubuntu1 all MAAS python API client (Python 3)
ii python3-maas-provisioningserver 2.4.2-7034-g2f5deb8b8-0ubuntu1 all MAAS server provisioning libraries (Python 3)
```