scons: Update comments in SConstruct.
scons: Update comments in SConstruct.

Make them consistently styled, gramatically and factually correct, and
try to make sure they're reasonably clear.

Change-Id: I3566734b0ac386113ef45ac7fd24c9fb1f2007d3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40972
Reviewed-by: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Reviewed-by: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Maintainer: Bobby R. Bruce <b9dba908ffb6c6d8ffe449cc88b3709c13c485c2@ucdavis.edu>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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