Added spec docs to app API public objects

lib/api/src/externals/mod.rs

@@ -15,15 +15,24 @@ use crate::store::{Store, StoreObject};
 use crate::ExternType;
 use wasmer_vm::Export;
 
+/// An external value is the runtime representation of an entity that
+/// can be imported or exported.
+///
+/// Spec: https://webassembly.github.io/spec/core/exec/runtime.html#external-values
 #[derive(Clone)]
 pub enum Extern {
+    /// A external [`Function`]
     Function(Function),
+    /// A external [`Global`]
     Global(Global),
+    /// A external [`Table`]
     Table(Table),
+    /// A external [`Memory`]
     Memory(Memory),
 }
 
 impl Extern {
+    /// Return the undelying type of the inner `Extern`.
     pub fn ty(&self) -> ExternType {
         match self {
             Extern::Function(ft) => ExternType::Function(ft.ty().clone()),