stdlib,configs: Update riscvmatched-fs example docstring

This documentation string provided in the
"config/example/gem5_library/riscvmatched-fs.py" was minimal. This patch
adds more detail.

Change-Id: I0f203ea6952fc72a078594d7c30853bd426017ff
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/65851
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
1 file changed