Revise specific guidance about avoiding 'CPU'

[hugovk]

Fixes python/docs-community#109.

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📚 Documentation preview 📚: https://cpython-devguide--1289.org.readthedocs.build/

[gvanrossum]

I would prefer to leave the entry for CPU and just state something like "This is a well-known abbreviation and is spelled in all uppercase without periods."

[willingc]

@hugovk In case it is helpful. Here's the language that I shared with the Editorial Board last month:

Allow use of CPU in text. It is common in today's language. There is no need to expand to "Central Processing Unit" or "processor" or define the term.

[hugovk]

Thanks, I condensed those down to:

CPU
   Central processing unit. No need to spell out.

[willingc]

Thanks @hugovk. 🚢