Improve E0137 error explanatIon

Author: GuillaumeGomez

src/librustc/diagnostics.rs

@@ -392,9 +392,30 @@ function `main()`. If there are multiple such functions, please rename one.
 "##,
 
 E0137: r##"
+More than one function was declared with the `#[main]` attribute.
+
+Erroneous code example:
+
+```compile_fail
+#![feature(main)]
+
+#[main]
+fn foo() {}
+
+#[main]
+fn f() {} // error: multiple functions with a #[main] attribute
+```
+
 This error indicates that the compiler found multiple functions with the
 `#[main]` attribute. This is an error because there must be a unique entry
-point into a Rust program.
+point into a Rust program. Example:
+
+```
+#![feature(main)]
+
+#[main]
+fn f() {} // ok!
+```
 "##,
 
 E0138: r##"