Add E0103 error explanation

Author: GuillaumeGomez

src/librustc_typeck/diagnostics.rs

@@ -1278,6 +1278,15 @@ fn main() {
 ```
 "##,
 
+E0103: r##"
+You hit this error because the compiler the compiler lacks information
+to determine a type for this pattern binding.
+
+You have two possibilities to solve this situation:
+ * Give an explicit definition of the expression
+ * Infer the expression
+"##,
+
 E0106: r##"
 This error indicates that a lifetime is missing from a type. If it is an error
 inside a function signature, the problem may be with failing to adhere to the
@@ -2701,7 +2710,6 @@ register_diagnostics! {
     E0085,
     E0086,
     E0090,
-    E0103,
     E0104,
     E0118,
     E0122,