Clean up err codes

src/librustc_error_codes/error_codes/E0130.md

@@ -1,4 +1,4 @@
-You declared a pattern as an argument in a foreign function declaration.
+A pattern was declared as an argument in a foreign function declaration.
 
 Erroneous code example:
 
@@ -9,7 +9,7 @@ extern {
 }
 ```
 
-Please replace the pattern argument with a regular one. Example:
+To fix this error, replace the pattern argument with a regular one. Example:
 
 ```
 struct SomeStruct {

src/librustc_error_codes/error_codes/E0131.md

@@ -1,8 +1,11 @@
-It is not possible to define `main` with generic parameters.
-When `main` is present, it must take no arguments and return `()`.
+The `main` function was defined with generic parameters.
+
 Erroneous code example:
 
 ```compile_fail,E0131
 fn main<T>() { // error: main function is not allowed to have generic parameters
 }
 ```
+
+It is not possible to define the `main` function with generic parameters.
+It must not take any arguments.