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This patch adds a mention of OptionalOrNullptr to the optional.md docs.

Change-Id: I7d09afa336bcd06a11756b5d655370d162b80318
Reviewed-on: https://chromium-review.googlesource.com/965585Reviewed-by: danakj <danakj@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543725}