Add a comment documenting fuel consumption rates.
@@ -574,6 +574,11 @@ impl Store {
/// immediately trap). This function must be called for the store to have
/// some fuel to allow WebAssembly to execute.
///
+ /// Most WebAssembly instructions consume 1 unit of fuel. Some
+ /// instructions, such as `nop`, `drop`, `block`, and `loop`, consume 0
+ /// units, as any execution cost associated with them involves other
+ /// instructions which do consume fuel.
+ ///
/// Note that at this time when fuel is entirely consumed it will cause
/// wasm to trap. More usages of fuel are planned for the future.
///
