Adding Too New to Rate Guidance to the Handbook and update SPOS to SPOF

Why is this change being made?

Aligned with the Too New to Rate proposal: https://docs.google.com/document/d/1suvZtb0j9zNVKyV46G1dBDjM6W7NfBQdQ5oo2ijjuGI/edit adding the guidance to the handbook.

This also changes "SPOS" to "SPOF"

@gl-peoplepartners for your review.

Author and Reviewer Checklist

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