docs: add POST for document update handlers along with deprecation notice

[dominicbarnes]

Overview

This fixes #4631 by copying the documentation for PUT /{db}/_design/{ddoc}/_update/{func}/{docid} as POST /{db}/_design/{ddoc}/_update/{func}/{docid}, but with a deprecation notice:

This has been supported historically, but is deprecated in favor of PUT.

Screenshot of the relevant section from my locally-built version of src/docs/.

Related Issues or Pull Requests

#4631

Checklist

• This is my own work, I did not use AI, LLM's or similar technology
• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• Documentation changes were made in the src/docs folder
• Documentation changes were backported (separated PR) to affected branches