Document junit_family option in the reference docs

[nicoddemus]

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[codecov]

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@@            Coverage Diff            @@
##             master    #4743   +/-   ##
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[nicoddemus]

@RonnyPfannschmidt @asottile @blueyed gentle ping. 😁

[asottile]

[asottile]

(oddly enough, we hit this at lyft recently and I was struggling to find the docs -- good to know I wasn't losing it!)