Document Runnable resource limits & behavior

[flaki]

Came up as part of suborbital/devrel#29 and this resulting Discord discussion.

We use the [resource limit] defaults for the individual runtimes, and that is something that needs to be documented

We should document the fact that Reactr uses the default resource limits of the WebAssembly runtime used under the hood for things like memory allocation limits. Ideally we should also collect and document what these limits or behaviors are (or at least link to the respective places where these are documented).