Document the tools directory (#6228)

As in https://github.com/astral-sh/uv/pull/6227

crates/uv-cli/src/lib.rs

@@ -2713,11 +2713,15 @@ pub enum ToolCommand {
     UpdateShell,
     /// Show the path to the uv tools directory.
     ///
-    /// The tools directory is used to store environments and metadata for
-    /// installed tools.
+    /// The tools directory is used to store environments and metadata for installed tools.
     ///
-    /// To instead view the directory uv installs executables into, use the
-    /// `--bin` flag.
+    /// By default, tools are stored in the uv data directory at `$XDG_DATA_HOME/uv/tools` or
+    /// `$HOME/.local/share/uv/tools` on Unix and `{FOLDERID_RoamingAppData}\uv\data\tools` on
+    /// Windows.
+    ///
+    /// The tool installation directory may be overridden with `$UV_TOOL_DIR`.
+    ///
+    /// To instead view the directory uv installs executables into, use the `--bin` flag.
     Dir(ToolDirArgs),
 }