Document NameClient class usage

from discussion at https://groups.google.com/a/chromium.org/d/msg/platform-architecture-dev/AP--F5L8lBY/Ms-VoV9dDAAJ

Bug: None
Change-Id: I5f733cc5cd74c9e658de7d528a3f7f1fa2537c59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1687719
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#674936}
1 file changed