MINOR: Clarify impact of num.replica.fetchers

[joel-hamill]

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[joel-hamill]

cc @guozhangwang @hachikuji

[divijvaidya]

@hachikuji one of the flaky failing tests in this PR is Build / JDK 11 and Scala 2.13 / testListenerConnectionRateLimitWhenActualRateAboveLimit() – kafka.network.ConnectionQuotasTest I have fixed that test at #12045

Please take a look when you get a chance.

[hachikuji]

Thanks, LGTM.