docs(pm): note why tool.uv.package = true makes a member buildable

Review suggestion from #122098: spell out on the condition line that
uv package mode produces real build metadata, so such members keep
their declared name.

pm/workspace.py

@@ -236,6 +236,7 @@ def _workspace_member(plugin_dir: Path, root: Path, *, identity: Path) -> Path:
         # the unique key in its name, as manifest-only members already do. A
         # buildable member keeps its declared name — uv verifies it against the
         # package metadata its backend produces.
+        # tool.uv.package = true opts into uv package mode: real build metadata, so buildable.
         virtual = "build-system" not in document and document.get("tool", {}).get("uv", {}).get("package") is not True
         changed = declaration.install_requirements != declaration.requirements
         if changed: