edit E0453 long explanation for style, clarity, and citizenship

It turns out that the subsequent lines of the error message comment
should be aligned like this.

The "turns the corresponding compiler warning" language may not be
strictly the most accurate—a lint check isn't the same as a compiler
warning; it emits a compiler warning if it's set to the `warn` level—
but it may be worth glossing over such distinctions in favor of simple,
familar phrasings for the sake of pedagogy; thanks to Guillaume Gomez
for the wording suggestion.

Let's also fix up the introductory clauses of the sentences about how to
fix the error to put a little more emphasis on the fact that the
`forbid` setting was probably there for a reason.

src/librustc/diagnostics.rs

@@ -1456,17 +1456,17 @@ Example of erroneous code:
 #[allow(non_snake_case)]
 fn main() {
     let MyNumber = 2; // error: allow(non_snake_case) overruled by outer
-                      // forbid(non_snake_case)
+                      //        forbid(non_snake_case)
 }
 ```
 
-The `forbid` lint setting makes code that fails the lint check result in a
-compilation-terminating error (like `deny`), but also prevents itself from
-being overridden by inner attributes.
+The `forbid` lint setting, like `deny`, turns the corresponding compiler
+warning into a hard error. Unlike `deny`, `forbid` prevents itself from being
+overridden by inner attributes.
 
-You can change `forbid` to `deny` (or use `-D` instead of `-F` if the `forbid`
-setting was given as a command-line option) to allow the inner lint check
-attribute:
+If you're sure you want to override the lint check, you can change `forbid` to
+`deny` (or use `-D` instead of `-F` if the `forbid` setting was given as a
+command-line option) to allow the inner lint check attribute:
 
 ```
 #![deny(non_snake_case)]
@@ -1477,7 +1477,7 @@ fn main() {
 }
 ```
 
-Alternatively, edit the code to pass the lint check, and remove the overruled
+Otherwise, edit the code to pass the lint check, and remove the overruled
 attribute:
 
 ```