Update the description of fetches-per-zone counters

(cherry picked from commit 65f6d8af)
79 jobs for !5604 with 2850-the-list-of-fetches-at-the-end-of-rndc-recursing-output-is-very-poorly-explained-in-the-arm-v9_16 in 18 minutes and 25 seconds (queued for 13 seconds)
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