Document that opcodes.rs contains InstructionData.

lib/cretonne/src/ir/instructions.rs

@@ -31,6 +31,7 @@ pub type ValueListPool = entity::ListPool<Value>;
 // Include code generated by `lib/cretonne/meta/gen_instr.py`. This file contains:
 //
 // - The `pub enum InstructionFormat` enum with all the instruction formats.
+// - The `pub enum InstructionData` enum with all the instruction data fields.
 // - The `pub enum Opcode` definition with all known opcodes,
 // - The `const OPCODE_FORMAT: [InstructionFormat; N]` table.
 // - The private `fn opcode_name(Opcode) -> &'static str` function, and