rec: document potential upgrade issue

[omoerbeek]

Short description

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 10787928321

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 50 unchanged lines in 11 files lost coverage.
• Overall coverage increased (+0.03%) to 64.692%

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Files with Coverage Reduction	New Missed Lines	%
pdns/backends/gsql/gsqlbackend.hh	1	97.71%
pdns/iputils.cc	3	55.3%
pdns/packethandler.cc	3	72.56%
pdns/fstrm_logger.cc	3	44.08%
pdns/recursordist/syncres.cc	3	79.51%
pdns/recursordist/recpacketcache.hh	3	89.55%
pdns/rcpgenerator.cc	3	89.77%
pdns/signingpipe.cc	5	83.33%
pdns/recursordist/test-syncres_cc1.cc	8	89.51%
pdns/dnsdistdist/dnsdist-tcp.cc	8	75.79%

Totals
Change from base Build 10787838361:	0.03%
Covered Lines:	124839
Relevant Lines:	162254

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💛 - Coveralls