docs: update zygote documentation

Follows up from:
[chromium-dev] Is it possible to get rid of the zygote process?
https://groups.google.com/a/chromium.org/d/msg/chromium-dev/xqSV4C60VyQ/qx0_KjVHBQAJ

[email protected]

Review-Url: https://codereview.chromium.org/2681483003
Cr-Commit-Position: refs/heads/master@{#448633}
1 file changed