Formalize preferred Author dates format (YYYY-MM-DD)

### Feature Request

### Problem / Opportunity
Now that the Author search is sortable by birth date, it works best when the dates are in YYYY-MM-DD format, so the instructions on the edit page should be updated to reflect that.

The "Sort by Birth Date" function currently will sort by whatever numbers lead the field, so dates stored in the suggested DD MON YYYY format will put those authors first regardless of birth year:

The instructions on the Author edit page should be updated to reflect a preferred YYYY-MM-DD format.

### Proposal

### Breakdown

### Related files

Refer to [this map of common Endpoints](https://github.com/internetarchive/openlibrary/wiki/Endpoints):
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### Requirements Checklist
Checklist of requirements that need to be satisfied in order for this issue to be closed:
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### Stakeholders

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