Improve description of mirror zone validation
This MR expands the description of mirror zones in the ARM after the bits about IXFR processing were removed in !1544 (merged).
It consists of the following commits:
4d43243a: mostly whitespace tweaks with a few insignificant text updates (hint: use
git show --color-wordsto fish these out from this commit),
92046a70: move a paragraph,
ef048b78: describe the mirror zone validation process.
All of these should be squashed into a single commit before merging; they are only left as separate changes to help the reviewer figure out what is happening.
Closes #1802 (closed)