Update usage paragraph of next/script onLoad

[lfades]

Changing the paragraph to not include beforeInteractive as one of the possible use cases of onLoad.

Update: Added docs for onError in the API reference of next/script.

@housseindjirdeh does onError also has the same limitation or is this only for onLoad?

Closes #33402

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[housseindjirdeh]

Thanks for this @lfades 🙏

Both onLoad and onError have the same limitations (they can only be used with afterInteractive and lazyOnload). Might be worthwhile to specify the same limitation for onError in this PR as well.

[lfades]

Updated the PR to include a note for both methods, and also added the docs for onError to the API reference as it was missing there.

[ijjk]

Thanks!