docs(lex): Make a determination on to_long_stdio

This tempts me to drop our design philosophy but I want to give it more
time.

clap_lex/src/lib.rs

@@ -292,6 +292,9 @@ impl<'s> ParsedArg<'s> {
     /// Treat as a long-flag
     ///
     /// **NOTE:** May return an empty flag.  Check [`ParsedArg::is_escape`] to separately detect `--`.
+    ///
+    /// **NOTE:** Will not match [`ParsedArg::is_stdio`], completion engines will need to check
+    /// that case.
     pub fn to_long(&self) -> Option<(Result<&str, &RawOsStr>, Option<&RawOsStr>)> {
         if let Some(raw) = self.utf8 {
             let remainder = raw.strip_prefix("--")?;

clap_lex/tests/parsed.rs

@@ -1,16 +1,15 @@
+// Despite our design philosophy being to support completion generation, we aren't considering `-`
+// the start of a long because there is no valid value to return.
 #[test]
-#[should_panic] // Our design philosophy is to match X if it can be completed as X which we break here
 fn to_long_stdio() {
     let raw = clap_lex::RawArgs::from_iter(["bin", "-"]);
     let mut cursor = raw.cursor();
     assert_eq!(raw.next_os(&mut cursor), Some(std::ffi::OsStr::new("bin")));
     let next = raw.next(&mut cursor).unwrap();
 
-    assert!(next.is_long());
+    assert!(!next.is_long());
 
-    let (key, value) = next.to_long().unwrap();
-    assert_eq!(key, Ok(""));
-    assert_eq!(value, None);
+    assert_eq!(next.to_long(), None);
 }
 
 #[test]