Also reword the testing/quick.Config field docs to conform to the
normal subject-first style. Without that style, godoc links
/pkg/testing/quick/#Config.Rand to the wrong line, since it doesn't
recognize the preceding comment as necessarily being attached.

Fixes #20809

Change-Id: I9aebbf763eed9b1ab1a153fa11850d88a65571c6
Reviewed-on: https://go-review.googlesource.com/46910Reviewed-by: Ian Lance Taylor <iant@golang.org>