[v9_16] Improve description of mirror zone validation
Expand the description of mirror zones in the ARM by adding a brief discussion of how the validation process works for AXFR and IXFR. Move the paragraph mentioning the "file" option higher up. Apply minor stylistic and whitespace-related tweaks to the relevant section of the ARM.
(cherry picked from commit d877aa9a)
Closes #1802 (closed)