Explain lack of reproducibility for @DisabledUntil

[nipafx]

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[nipafx]

Proposed commit message:

Explain lack of reproducibility for `@DisabledUntil` (#550 / #645)

A test suite that only passes because a failing test is disabled
until a certain date, will fail if executed after that date. That
means the originally successful build can't be reproduced.

We're looking for a proper solution for this (see #478), but until
then, the best we can do is warn users about this in documentation
and with a report entry.

References: #478, #550
PR: 645