Improve the documentation for the chrome.runtime.requestUpdateCheck API
Improve the documentation for the chrome.runtime.requestUpdateCheck API

Make it clear that this method is for very limited circumstances and
shouldn't normally be used.

Review URL: https://codereview.chromium.org/1908213002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#388968}

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