Document that fish only supports named args

src/build/arg/value_hint.rs

@@ -6,7 +6,7 @@ use std::str::FromStr;
 ///
 /// Overview of which hints are supported by which shell:
 ///
-/// | Hint                   | zsh | fish    |
+/// | Hint                   | zsh | fish[^1]|
 /// | ---------------------- | --- | ------- |
 /// | `AnyPath`              | Yes | Yes     |
 /// | `FilePath`             | Yes | Yes     |
@@ -19,6 +19,9 @@ use std::str::FromStr;
 /// | `Hostname`             | Yes | Yes     |
 /// | `Url`                  | Yes |         |
 /// | `EmailAddress`         | Yes |         |
+///
+/// [^1]: fish completions currently only support named arguments (e.g. -o or --opt), not
+///       positional arguments.
 #[derive(Debug, PartialEq, Copy, Clone)]
 pub enum ValueHint {
     /// Default value if hint is not specified. Follows shell default behavior, which is usually