Document immediate-trap behavior of epochs (#4345)

Closes #4342

crates/wasmtime/src/config.rs

@@ -449,9 +449,9 @@ impl Config {
     /// not available. This means epoch-based interruption can only
     /// serve as a simple external-interruption mechanism.
     ///
-    /// An initial deadline can be set before executing code by
-    /// calling
-    /// [`Store::set_epoch_deadline`](crate::Store::set_epoch_deadline).
+    /// An initial deadline must be set before executing code by calling
+    /// [`Store::set_epoch_deadline`](crate::Store::set_epoch_deadline). If this
+    /// deadline is not configured then wasm will immediately trap.
     ///
     /// ## When to use fuel vs. epochs
     ///

crates/wasmtime/src/store.rs

@@ -870,6 +870,10 @@ impl<T> Store<T> {
     /// [`Engine::increment_epoch()`] has been invoked at least
     /// `ticks_beyond_current` times.
     ///
+    /// By default a store will trap immediately with an epoch deadline of 0
+    /// (which has always "elapsed"). This method is required to be configured
+    /// for stores with epochs enabled to some future epoch deadline.
+    ///
     /// See documentation on
     /// [`Config::epoch_interruption()`](crate::Config::epoch_interruption)
     /// for an introduction to epoch-based interruption.
@@ -896,6 +900,10 @@ impl<T> Store<T> {
     /// finite interval. For deterministic interruption, see the
     /// "fuel" mechanism instead.
     ///
+    /// Note that when this is used it's required to call
+    /// [`Store::set_epoch_deadline`] or otherwise wasm will always immediately
+    /// trap.
+    ///
     /// See documentation on
     /// [`Config::epoch_interruption()`](crate::Config::epoch_interruption)
     /// for an introduction to epoch-based interruption.