fix(complete): Have shorts fallback to using long as description

It should be rare that this is helpful

clap_complete/src/engine/complete.rs

@@ -447,7 +447,11 @@ fn shorts_and_visible_aliases(p: &clap::Command) -> Vec<CompletionCandidate> {
             a.get_short_and_visible_aliases().map(|shorts| {
                 shorts.into_iter().map(|s| {
                     CompletionCandidate::new(s.to_string())
-                        .help(a.get_help().cloned())
+                        .help(
+                            a.get_help()
+                                .cloned()
+                                .or_else(|| a.get_long().map(|long| format!("--{long}").into())),
+                        )
                         .id(Some(format!("arg::{}", a.get_id())))
                         .hide(a.is_hide_set())
                 })

clap_complete/tests/testsuite/engine.rs

@@ -1114,8 +1114,8 @@ pos-c
     assert_data_eq!(
         complete!(cmd, "-[TAB]"),
         snapbox::str![[r#"
--r
--o
+-r	--required-flag
+-o	--optional-flag
 -s
 -h	Print help
 --long-flag