docs: mention non-specific method exception codes

[Ehesp]

Description

Mention non-specific method exception codes in the documentation.

Related Issues

Fixes #3273

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[github-actions]

Deploy preview for 7dd254d:

https://flutter-firebase-docs--pr3317-docs-3273-bhi8ins1.web.app

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[feinstein]

This PR doesn't include changes into FirebaseAuthException as requested by @Ehesp in #3273 here.

[Salakar]

Reopened original issue to track, it got automatically closed when this one was merged 🙈