Clean up E0750 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0750.md

@@ -1,4 +1,18 @@
-Negative impls cannot be default impls. A default impl supplies
-default values for the items within to be used by other impls, whereas
-a negative impl declares that there are no other impls. These don't
-make sense to combine.
+A negative impl was made default impl.
+
+Erroneous code example:
+
+```compile_fail,E0750
+# #![feature(negative_impls)]
+# #![feature(specialization)]
+trait MyTrait {
+    type Foo;
+}
+
+default impl !MyTrait for u32 {} // error!
+# fn main() {}
+```
+
+Negative impls cannot be default impls. A default impl supplies default values
+for the items within to be used by other impls, whereas a negative impl declares
+that there are no other impls. Combining it does not make sense.