Small error explanations cleanup

src/librustc_error_codes/error_codes/E0136.md

@@ -1,5 +1,4 @@
-A binary can only have one entry point, and by default that entry point is the
-function `main()`. If there are multiple such functions, please rename one.
+More than one `main` function was found.
 
 Erroneous code example:
 
@@ -14,3 +13,7 @@ fn main() { // error!
     // ...
 }
 ```
+
+A binary can only have one entry point, and by default that entry point is the
+`main()` function. If there are multiple instances of this function, please
+rename one of them.

src/librustc_error_codes/error_codes/E0161.md

@@ -1,5 +1,4 @@
-A value was moved. However, its size was not known at compile time, and only
-values of a known size can be moved.
+A value was moved whose size was not known at compile time.
 
 Erroneous code example: