Clarify language detection module level documentation.

Workshopped wording with aboxhall@.

R=aboxhall,akihiroota

Change-Id: I40fa56ea503fde2e55f7aa55b70004fc83297951
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1692383
Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Commit-Queue: Chris Hall <chrishall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675925}