Update E0423 description

E0423 doesn't apply only to structs, update the error index description
to make this clear.

src/librustc_resolve/diagnostics.rs

@@ -967,16 +967,18 @@ one.
 "##,
 
 E0423: r##"
-A `struct` variant name was used like a function name.
+An identifier was used like a function name or a value was expected and the
+identifier exists but it belongs to a different namespace.
 
-Erroneous code example:
+For (an erroneous) example, here a `struct` variant name were used as a
+function:
 
 ```compile_fail,E0423
 struct Foo { a: bool };
 
 let f = Foo();
-// error: `Foo` is a struct variant name, but this expression uses
+// error: expected function, found `Foo`
-//        it like a function name
+// `Foo` is a struct name, but this expression uses it like a function name
 ```
 
 Please verify you didn't misspell the name of what you actually wanted to use
@@ -987,6 +989,30 @@ fn Foo() -> u32 { 0 }
 
 let f = Foo(); // ok!
 ```
+
+It is common to forget the trailing `!` on macro invocations, which would also
+yield this error:
+
+```compile_fail,E0423
+println("");
+// error: expected function, found macro `println`
+// did you mean `println!(...)`? (notice the trailing `!`)
+```
+
+Another case where this error is emitted is when a value is expected, but
+something else is found:
+
+```compile_fail,E0423
+pub mod a {
+    pub const I: i32 = 1;
+}
+
+fn h1() -> i32 {
+    a.I
+    //~^ ERROR expected value, found module `a`
+    // did you mean `a::I`?
+}
+```
 "##,
 
 E0424: r##"