Further expand the explanation for exemptions to 3.1.4 Abbreviations

[patrickhlauke]

Follow-up to #4815 based on @chaals comment:

There's a small class of common abbreviations that are generally recognised as such, but are far more commonly written in their abbreviated forms. Things like "dr", "etc" and "mrs" are I think common across english varieties, while there are local expressions such as "Plc" (UK - doesn't translate so easily to other countries) or "Jr" and "Sr" used in names far more in the US than other large countries in the anglosphere.

I think the intent is to cover these abbreviations as allowable parts of the language that do not require an expansion to be provided.

I suspect it is also intended to cover certain terms from chemistry that are geenrally used as words, such as CFC, PVC, PFA, and maybe very common biological terms like DNA or RNA.

"Rep." is commonly used in the US, to mean either "representative" as in a member of the US House, or "republican" as in a member of the Republican party. given the potential ambiguity I think it's clear this isn't intended to be accepted as an exemption.

I suspect the same applies to "MP", used in commonwealth countries to mean "Member of Parliament".

I believe the intention is that organisational terms (Lt., SVP, PFC, Ass., Sr.), and technical acronyms used in commerce (FHE, P/L, RSJ, ROI, VPAT, RBAC, TVL, ABS), are not exempt.

What about such terms as "MLK", "JFK" or "FDR"? Although they are essentially local US terms, they have very high recognition...

Potentially also consider if this clarification (and the ones from #4815) wouldn't make more sense as notes to the normative term definition of abbreviation

[patrickhlauke]

from @chaals

I think the "intended audience" approach tends to be used to say "but everyone who is meant to read this understands P/L, TVL and FHE ... ", which I don't think is the point.

It is more that there are a handful of abbreviations recognised as such, but used for so long and so commonly that people know what they mean, but don't know how to write them in full ("Mrs" which dates back centuries or "Ms" which is about as old as me so has been used since before most people alive learned to read and write - how do you write either of these in unabbreviated form?).

More concrete guidance on what isn't intended to be exempt would be helpful. Happy if that's bumped to a different PR.