Update lucet-runtime/lucet-runtime-internals/src/instance/execution.rs
//! `NotTerminable` when the instance has already been terminated.
//! * `Instance::run returns`
//! - terminable: `true`
-//! - execution_domain: `Guest, Hostcall, or Terminated`
+//! - execution_domain: `Pending, Hostcall, or Terminated`
//! - termination result: `Err(KillError::Invalid)`
//! - `execution_domain` will be `Pending` when the initial guest function returns, `Hostcall`
//! when terminated by `lucet_hostcall_terminate!`, and `Terminated` when exiting due to a
