Clarify documentation on updating PO files

What does this MR do and why?

In my first MR editing externalized strings, I struggled to figure out how to update translation files; there's a lot of documentation about adding translation for other languages, but nowhere in the i18n docs does it say "hey by the way, if you edit these strings in the American English version, here's how you update the PO files". I am adding that here.

Related to Update lefthook error message for gettext (!137182 - merged)

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