Fix uv python install --default note about multiple requests (#10011)

Author: zanieb

crates/uv-cli/src/lib.rs

@@ -4354,8 +4354,7 @@ pub struct PythonInstallArgs {
     /// 3.13+freethreaded with `--default` will include in `python3t` and `pythont`, not `python3`
     /// and `python`.
     ///
-    /// If multiple Python versions are requested during the installation, the first request will be
-    /// the default.
+    /// If multiple Python versions are requested, uv will exit with an error.
     #[arg(long)]
     pub default: bool,
 }

crates/uv/tests/it/help.rs

@@ -540,8 +540,7 @@ fn help_subsubcommand() {
               3.13+freethreaded with `--default` will include in `python3t` and `pythont`, not `python3`
               and `python`.
               
-              If multiple Python versions are requested during the installation, the first request will
-              be the default.
+              If multiple Python versions are requested, uv will exit with an error.
 
     Cache options:
       -n, --no-cache

docs/reference/cli.md

@@ -4709,7 +4709,7 @@ uv python install [OPTIONS] [TARGETS]...
 
 <p>Alternative Python variants will still include their tag. For example, installing 3.13+freethreaded with <code>--default</code> will include in <code>python3t</code> and <code>pythont</code>, not <code>python3</code> and <code>python</code>.</p>
 
-<p>If multiple Python versions are requested during the installation, the first request will be the default.</p>
+<p>If multiple Python versions are requested, uv will exit with an error.</p>
 
 </dd><dt><code>--directory</code> <i>directory</i></dt><dd><p>Change to the given directory prior to running the command.</p>