core(i18n): reframe the ICU message descriptions with placeholders

[paulirish]

First, a word from the docs:

The industry standard, ICU MessageFormat Syntax, is used to format these messages internally. However, only select parts of that syntax work at Google.

lol thx. So #5736 was a waste and doesn't work. So we'll describe the placeholders this way.

[patrickhulce]

However, only select parts of that syntax work at Google.

doh! 🤦‍♂️

lgtm! though I wonder if we should also call out {placehoder}\xa0 as the special thing to not touch just so they don't get thrown by the character code :)