Pages: Clarify persistence of custom domain verification

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The Pages domain interface does not mention that the verification TXT entry has to remain even after verification. This should be mentioned in this interface, because removing the entry after verification is, at least to me, the natural thing to do.

This is documented on this page, but the Pages interface is easy enough to use that most users probably won't read it, the first time they notice their mistake is when they receive the email that further validation has failed.

cc @marcia because this may be relevant for gitlab-ce#39978?