docs(libanki): defaultsForAdding & defaultDeckForNoteType

[david-allison]

Note

Assisted-by: GPT-6 (with lots of human guidance)

Fixes

• Part of Deck Selection is not persistent between adding notes #20497 - before I fix it, I want the issue to be easily understandable.

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[criticalAY]

LGTM!