Update variable.rs

cranelift/frontend/src/variable.rs

@@ -2,6 +2,11 @@
 //!
 //! Frontends can use any indexing scheme they see fit and
 //! generate the appropriate `Variable` instances.
+//!
+//! Note: The `Variable` is used by Cranelift to index an array containing
+//! information about your mutable variables. Thus, when you create a new
+//! `Variable` you should make sure that the index is provided by a counter
+//! incremented by 1 each time you encounter a new mutable variable.
 
 use core::u32;
 use cranelift_codegen::entity::EntityRef;