fix(derive): Set both about/long_about with doc comments

The main care about is that when we override a `flatten` / `subcommand`
doc comment in a parent container, that we make sure we take nothing
from the child container, rather than implicitly taking one `about` ut
not `long_about`.

To do this, and to play the most safe with long help detection, we reset
`long_about` to default when there is no doc comment body to use for
`long_about`.

Fixes #2893

clap_derive/src/utils/doc_comments.rs

@@ -61,7 +61,10 @@ pub fn process_doc_comment(lines: Vec<String>, name: &str, preprocess: bool) ->
             lines.join("\n")
         };
 
-        vec![Method::new(short_name, quote!(#short))]
+        vec![
+            Method::new(short_name, quote!(#short)),
+            Method::new(long_name, quote!(None)),
+        ]
     }
 }
 

tests/derive/doc_comments_help.rs

@@ -234,8 +234,7 @@ fn doc_comment_about_handles_both_abouts() {
 
     let app = Opts::into_app();
     assert_eq!(app.get_about(), Some("Opts doc comment summary"));
-    assert_eq!(
-        app.get_long_about(),
-        Some("Sub doc comment summary\n\nSub doc comment body")
-    );
+    // clap will fallback to `about` on `None`.  The main care about is not providing a `Sub` doc
+    // comment.
+    assert_eq!(app.get_long_about(), None);
 }