Clean up E0510 explanation

src/librustc_error_codes/error_codes/E0510.md

@@ -1,16 +1,29 @@
-Cannot mutate place in this match guard.
+The matched value was assigned in a match guard.
 
-When matching on a variable it cannot be mutated in the match guards, as this
-could cause the match to be non-exhaustive:
+Erroneous code example:
 
 ```compile_fail,E0510
 let mut x = Some(0);
 match x {
-    None => (),
-    Some(_) if { x = None; false } => (),
-    Some(v) => (), // No longer matches
+    None => {}
+    Some(_) if { x = None; false } => {} // error!
+    Some(_) => {}
 }
 ```
 
+When matching on a variable it cannot be mutated in the match guards, as this
+could cause the match to be non-exhaustive.
+
 Here executing `x = None` would modify the value being matched and require us
-to go "back in time" to the `None` arm.
+to go "back in time" to the `None` arm. To fix it, change the value in the match
+arm:
+
+```
+let mut x = Some(0);
+match x {
+    None => {}
+    Some(_) => {
+        x = None; // ok!
+    }
+}
+```