dnsdist: Better documentation of the zero-scope feature

[rgacogne]

Short description

Fixes #14476

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Pull Request Test Coverage Report for Build 10787969502

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2760 unchanged lines in 55 files lost coverage.
• Overall coverage decreased (-0.01%) to 64.689%

---

Files with Coverage Reduction	New Missed Lines	%
pdns/dnspacket.hh	1	94.44%
pdns/dnsdistdist/dnsdist.hh	1	84.24%
pdns/query-local-address.cc	2	90.43%
pdns/recursordist/aggressive_nsec.cc	2	66.39%
pdns/recursordist/rec-tcounters.hh	2	96.84%
pdns/recursordist/test-syncres_cc2.cc	3	88.85%
pdns/dnsparser.hh	3	78.27%
pdns/recursordist/lua-recursor4.hh	3	90.32%
pdns/fstrm_logger.cc	3	44.55%
pdns/tcpiohandler.cc	3	66.73%

Totals
Change from base Build 9991931844:	-0.01%
Covered Lines:	124846
Relevant Lines:	162248

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💛 - Coveralls