tests: update nightly tests to document square
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 <d2406197fb1bedd1e23c70c50c510ff0e9ab0f5b@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5