[manifest] Clarify launching from a deep link. (#301)

The spec currently says:

> When an application context is created, the user agent MUST immediately navigate to the start URL with replacement enabled.

This doesn't handle the case where an app needs to open to a URL that is within its scope (i.e., to handle "deep linking").  

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/301