Add note on resolution to typeRoots documentation

[wjhsf]

The default typeRoots config looks within the node_modules directory, (specifically, node_modules/@types) to resolve imports. This gives the impression that the contents of a custom typeRoots directory are treated as node_modules packages, and therefore can make use of features likepackage.json "exports" field. However, this does not seem to be the case. I couldn't find it explicitly documented anywhere, but through trial and error it seems that they are actually treated as directory modules. Hopefully, adding a note to the documentation will save some time for others looking into this.

[jakebailey]

I'm not sure that's exactly how it works; @types packages can have export maps and they work. Are you sure this isn't just a bug in TS?

[wjhsf]

I'm not sure that's exactly how it works; @types packages can have export maps and they work. Are you sure this isn't just a bug in TS?

I'm not sure! I couldn't find much information on typeRoots, so this was pretty much a guess based on some old comments on GitHub issues. I'd be happy to be wrong about this!