Tweak the description of the BackForwardCache feature.

Fix some grammar problems. Also mark "unload" as a non-translatable
term.

Change-Id: I422a2ad871a202c8576ad72b0166e8a4107dedab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2342536
Commit-Queue: Fergal Daly <fergal@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797250}