Correct description of E0502

Author: trevyn

compiler/rustc_error_codes/src/error_codes/E0502.md

@@ -1,4 +1,5 @@
-A variable already borrowed as immutable was borrowed as mutable.
+A variable already borrowed with a certain mutability (either mutable or
+immutable) was borrowed again with a different mutability.
 
 Erroneous code example:
 
@@ -13,7 +14,7 @@ fn foo(a: &mut i32) {
 ```
 
 To fix this error, ensure that you don't have any other references to the
-variable before trying to access it mutably:
+variable before trying to access it with a different mutability:
 
 ```
 fn bar(x: &mut i32) {}