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And adds basic explanation on the differences between the different
history services it houses.

Change-Id: Ic93603c9b2d9312315886a0fe10a4940c8b44829
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495907
Auto-Submit: Carlos Knippschild <carlosk@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821891}