Refine E0435 description

compiler/rustc_error_codes/src/error_codes/E0435.md

@@ -7,6 +7,12 @@ let foo = 42;
 let a: [u8; foo]; // error: attempt to use a non-constant value in a constant
 ```
 
+'constant' means 'a compile-time value'.
+
+More details can be found in the [Variables and Mutability] section of the book.
+
+[Variables and Mutability]: https://doc.rust-lang.org/book/ch03-01-variables-and-mutability.html#differences-between-variables-and-constants
+
 To fix this error, please replace the value with a constant. Example:
 
 ```