[url_launcher] Add note that canLaunch only works for some schemes

[tp]

And that some configuration is necessary to support custom schemes.

Description

Document that iOS apps need to be configured to allow querying for non-standard URL schemes.

Related Issues

• url_launcher not launching WhatsApp on iOS flutter#42311 (hopefully would have been clearer to the OP)

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[tp]

Please let me know if this is a good place to document this behavior, or if you would rather prefer a note on the README.

[stuartmorgan-g]

Thanks for the contribution! We apologize for the long delay in triaging this PR. We’re in the process of overhauling our PR triage system to respond much more quickly, as well as working through the backlog.

Since #3252 addresses this same concern (via the README, which is where we generally document permissions), and has already been reviewed, I'm going to go ahead and close this. Apologies for the unfortunate duplication due to this PR not having been triaged.