rec: better explain how the recurse field works in various contexts

[omoerbeek]

Short description

Plus an extra consistency check in the case of forward_zones (forward_zones_recurse cannot be done easily as the code actually does not look at the flag value, but is driven by the name of the field being processed, the incoming YAML has the flag set to false by default.

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)

[omoerbeek]

@kpfleming You might want to take a look at this PR

[coveralls]

Pull Request Test Coverage Report for Build 13201150258

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.
• 70 unchanged lines in 15 files lost coverage.
• Overall coverage increased (+0.6%) to 64.731%

---

Files with Coverage Reduction	New Missed Lines	%
pdns/recursordist/aggressive_nsec.cc	2	66.1%
pdns/backends/gsql/gsqlbackend.hh	2	97.71%
modules/lmdbbackend/lmdbbackend.cc	2	73.65%
pdns/ws-auth.cc	2	80.8%
pdns/recursordist/syncres.cc	3	80.05%
pdns/remote_logger.cc	3	53.81%
pdns/opensslsigners.cc	3	61.27%
pdns/recursordist/recpacketcache.hh	3	89.55%
pdns/rcpgenerator.cc	3	90.55%
pdns/recursordist/rec-tcp.cc	4	66.54%

Totals
Change from base Build 13196902717:	0.6%
Covered Lines:	128269
Relevant Lines:	167091

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💛 - Coveralls