Make help docs platform specific (#40194)

Julia Ryan and David Kleingeld created 

No need to clutter the `--help` docs with default directories for
platforms other than the current one.

Release Notes:

- N/A

Co-authored-by: David Kleingeld <davidsk@zed.dev>

Change summary

crates/cli/src/main.rs | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)

Detailed changes

crates/cli/src/main.rs 🔗 

@@ -68,10 +68,13 @@ struct Args {
     #[arg(short, long, overrides_with = "add")]
     new: bool,
     /// Sets a custom directory for all user data (e.g., database, extensions, logs).
-    /// This overrides the default platform-specific data directory location.
-    /// On macOS, the default is `~/Library/Application Support/Zed`.
-    /// On Linux/FreeBSD, the default is `$XDG_DATA_HOME/zed`.
-    /// On Windows, the default is `%LOCALAPPDATA%\Zed`.
+    /// This overrides the default platform-specific data directory location:
+    #[cfg_attr(target_os = "macos", doc = "`~/Library/Application Support/Zed`.")]
+    #[cfg_attr(target_os = "windows", doc = "`%LOCALAPPDATA%\\Zed`.")]
+    #[cfg_attr(
+        not(any(target_os = "windows", target_os = "macos")),
+        doc = "`$XDG_DATA_HOME/zed`."
+    )]
     #[arg(long, value_name = "DIR")]
     user_data_dir: Option<String>,
     /// The paths to open in Zed (space-separated).