Expand on activations table invariants comment in libcalls.rs

Author: fitzgen

Diff for: crates/runtime/src/libcalls.rs

@@ -406,7 +406,10 @@ pub unsafe extern "C" fn wasmtime_activations_table_insert_with_gc(
     // Invariant: all `externref`s on the stack have an entry in the activations
     // table. So we need to ensure that this `externref` is in the table
     // *before* we GC, even though `insert_with_gc` will ensure that it is in
-    // the table *after* the GC.
+    // the table *after* the GC. This technically results in one more hash table
+    // look up than is strictly necessary -- which we could avoid by having an
+    // additional GC method that is aware of these GC-triggering references --
+    // but it isn't really a concern because this is already a slow path.
     activations_table.insert_without_gc(externref.clone());
 
     activations_table.insert_with_gc(externref, module_info_lookup);