[resultdb][cli] chromium_derive is temporary
[resultdb][cli] chromium_derive is temporary

Document explicitly that chromium_derive subcommand is temporary.

Also note that it is "advanced", so it does not show up in `rdb help`
by default.

R=9e0728822fdb2ce07b011b42510312bb837e6a51@chromium.org

Change-Id: If5ee5f5f3ae2a6d0079871bf6a354684f88c5f34
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1949628
Auto-Submit: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Erik Staab <9e0728822fdb2ce07b011b42510312bb837e6a51@chromium.org>
Reviewed-by: Erik Staab <9e0728822fdb2ce07b011b42510312bb837e6a51@chromium.org>

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