[dispatcher] Fix+clarify options documentation.
[dispatcher] Fix+clarify options documentation.

Follow-up to https://chromium-review.googlesource.com/c/2861712.

R=ddoman, tandrii

Change-Id: I7ab1fef1548ec9533d059f4e6f69c9806590f15b
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2873430
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>
Auto-Submit: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Reviewed-by: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go