I just wanted to comment that it's pretty weird that there are now two documents called "Designing for Playdate" that are similar but seem to contain different information.
There's this document that's part of the SDK documentation and appears to be more comprehensive. However, it no longer seems to be linked from anywhere. Instead, the Dev resources page links to this new document, which is similar but seems to be missing some pieces of information.
In particular, I just spent twenty minutes searching the publicly linked docs for information on the Playdate control icon glyphs ( Ⓐ Ⓑ ⊙ ✛ ), which aren't documented in any of the public docs anymore.
[seinfield]What's the deal with all these documents?[/seinfield]
The hosted, Help-library version of Designing for Playdate should be considered the canonical one. We will remove the downloadable file and its online version. (It seems better to me to have canonical, online files for something like this.)
The only difference between them currently should be that note regarding glyphs. Unfortunately, that wasn't altogether accurate; those glyphs are present in some fonts, but not all, and they won't be substituted in by the default font correctly right now.
Probably the best way to see what characters are available in any given font is to open it in Caps.
⊙ 
✛ 
), which aren't documented in any of the public docs anymore.