Currently in our API `?` means null, but sometimes it means optional. Linting optional/nulls with this patch is required for #1166 to land nicely.
Previously, return types were not being linted in `api.md`. This is fixed, along with many broken return types.
This patch considers `?` to mean nullable, and has some heuristics to determine optionality. I believe this to be the minimal patch needed to unblock #1166. After it lands, we can consider changing the api docs to hopefully remove some heuristics and strangeness.
