ci(doc-linter): check every IP on a line, not just the first

`lint_ipv4()` and `lint_ipv6()` used `re.search`, which returns
only the first match. A line like

    Set DNS forwarder 192.0.2.1 then fall back to 8.8.8.8

flagged nothing because `192.0.2.1` (RFC 5737 documentation
range) is allowed and the search stopped there. The real
public IP `8.8.8.8` slipped through despite being exactly
the case the linter was meant to catch.

Switch both functions to `re.finditer` and walk every match:
return on the first disallowed address; only return None when
all matches on the line are allowed (private / multicast /
non-global).

Also fix the casing of "private space" in both error
messages — was "private Space" with a stray capital.

Verified with 7 hand-coded cases (allowed + public mixes,
boundary cases, IPv6 RFC 3849 / Google DNS). All pass.

Tracked as items 1, 2, and 10 of the rolling-side cleanup
backlog from PR #2014 / #2019 / #2020 reviews.

🤖 Generated by [robots](https://vyos.io)

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