tests: update nightly tests to document square

Add some information and comments on why square is included in the
nightly tests.

Change-Id: I80b61fb90f16ad0d693ec29975908549e8102382
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50949
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
1 file changed