Clean up E0590 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0590.md

@@ -1,13 +1,17 @@
-`break` or `continue` must include a label when used in the condition of a
-`while` loop.
+`break` or `continue` keywords were used in a condition of a `while` loop
+without a label.
 
-Example of erroneous code:
+Erroneous code code:
 
 ```compile_fail,E0590
 while break {}
 ```
 
+`break` or `continue` must include a label when used in the condition of a
+`while` loop.
+
 To fix this, add a label specifying which loop is being broken out of:
+
 ```
 'foo: while break 'foo {}
 ```