dnsdist: Document that DNSQuestion::getElapsedUs will be added in 1.9.8

[rgacogne]

Short description

As suggested by Otto in #14887

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 12238492354

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 54 unchanged lines in 14 files lost coverage.
• Overall coverage decreased (-0.02%) to 64.744%

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Files with Coverage Reduction	New Missed Lines	%
modules/gpgsqlbackend/gpgsqlbackend.cc	1	88.62%
pdns/recursordist/syncres.cc	1	79.38%
pdns/recursordist/recursor_cache.cc	1	84.13%
pdns/recursordist/sortlist.cc	2	74.12%
modules/lmdbbackend/lmdbbackend.cc	2	72.54%
pdns/dnsdistdist/dnsdist.cc	2	68.89%
pdns/recursordist/test-syncres_cc2.cc	3	88.91%
pdns/misc.hh	3	87.62%
pdns/remote_logger.cc	3	54.26%
pdns/recursordist/test-syncres_cc1.cc	6	89.66%

Totals
Change from base Build 12238306140:	-0.02%
Covered Lines:	125876
Relevant Lines:	163527

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💛 - Coveralls