Revise E0015 according to feedback.

Author: Nick Hamann

src/librustc/diagnostics.rs

@@ -196,9 +196,9 @@ const Y: i32 = A;
 "##,
 
 E0015: r##"
-The only function calls allowed in static or constant expressions are
-constructors for enum variants and structs, or `const fn`s. Rust currently does
-not support more general compile-time function execution.
+The only functions that can be called in static or constant expressions are
+`const` functions. Rust currently does not support more general compile-time
+function execution.
 
 See [RFC 911] for more details on the design of `const fn`s.