dashboard: reduce expected number of corellium builders to 2 each
dashboard: reduce expected number of corellium builders to 2 each

According to https://golang.org/issue/40523#issuecomment-676546824,
there are only 2 Android corellium hosts and 2 iOS corellium hosts.

Also add links to issues with more information to the builder notes.

Updates golang/go#40523.
Updates golang/go#31722.

Change-Id: Ibabccebfcc6735cba867cccf109c71607223d1a2
Reviewed-on: https://go-review.googlesource.com/c/build/+/249337
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>

golang/build,golang/build,golang/build,golang/build