Rename Maybe to Optional

And add examples of good and bad usage to the documentation.

R=aluebs@webrtc.org, henrik.lundin@webrtc.org, pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/1432553007 .

Cr-Original-Commit-Position: refs/heads/master@{#10588}
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Cr-Mirrored-Commit: be57983f4bd875c39a229bab5112b32dad004057
19 files changed