[ozone] Improve PlatformScreen documentation

Documentation for ui::PlatformScreen seemed incomplete to me while
deciding if an implementation was correct. Clarified.

Bug: 875161
Change-Id: Iceb8dd3fea2000606ccb92a58bba0b0e962b651b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1418820
Commit-Queue: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660491}
1 file changed