[docs] Refresh Developer Policy text (#136198)

Clarify lots of existing practice. Expand on the "major change" section,
which is the closest thing we have on how to run an RFC.

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Co-authored-by: Oleksandr "Alex" Zinenko <azinenko@amd.com>
Co-authored-by: Aaron Ballman <aaron@aaronballman.com>

Author: 3 people