Improve long explanation for E0542 and E0546

Author: jesusprubio

compiler/rustc_error_codes/src/error_codes/E0542.md

@@ -19,7 +19,7 @@ fn _stable_const_fn() {}
 fn _deprecated_fn() {}
 ```
 
-To fix the issue you need to provide the `since` field.
+To fix this issue, you need to provide the `since` field. Example:
 
 ```
 #![feature(staged_api)]

compiler/rustc_error_codes/src/error_codes/E0546.md

@@ -13,7 +13,7 @@ fn unstable_fn() {}
 fn stable_fn() {}
 ```
 
-To fix the issue you need to provide the `feature` field.
+To fix this issue, you need to provide the `feature` field. Example:
 
 ```
 #![feature(staged_api)]