Fix release notes for BIND 9.15.6
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Add a missing release note for TCP high-water. That feature was not yet merged when the initial version of !2524 (merged) was prepared and its release note was missed when that merge request was later rebased.
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Rephrase the release note for CVE-2019-6477 so that it uses the same text as its corresponding notes in all other releases.
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Unify whitespace in doc/arm/notes-9.15.6.xml.