Add details about how to handle all uppercase words

What does this MR do?

While reviewing a recent MR, a contributor mentioned that it wasn't clear how to handle things like protocols and other words that are all uppercase: !88873 (comment 965760013)

We've added guidelines for how to handle spelling exceptions (https://docs.gitlab.com/ee/development/documentation/testing.html#vale-spelling-test), and I think we should do the same for uppercase words/acronyms, to make it easier for our contributors to know how to handle these vale warnings.

This MR adds a new entry, following the same style as the spelling test guidelines above it, explaining how to handle vale warnings for all uppercase words/acronyms.

Related issues

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