simplify Google Docs Previews explanation, fixes CNVS-12777

test plan:
1. go to account settings
2. click the "?" next to Google Docs Previews
3. the explanation should be comprehensible to an average person (e.g.
   doesn't say "inline preview"), should not have grammatical
   errors, and should be all around less rambly

Change-Id: I5f4981c3316056bae51fa797b082c81d19f3f682
Reviewed-on: https://gerrit.instructure.com/34200
Reviewed-by: Dave Donahue <ddonahue@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>