Clarify that enum values are locale-neutral

[aphillips]

In reviewing compute-pressure's self review here I found that specdev doesn't explicitly talk about enumerated values. Section 7.2 does talk about application internal identifiers and recommends case-insensitive ASCII for these as well as wrapping with display strings. But explicit guidance in that section as well as a specific callout in Section 9 near this requirement seems appropriate.

The short checklist should also provide a nudge that enum values are an appropriate design pattern.

[aphillips]

Borrowed the text from the compute-pressure self-review (above) and included this in #121