add admonitions for macOS 15 Sequoia update (#1542)

Same premise/language as NixOS/nix#11487

The impending release of macOS 15 Sequoia will break many existing nix
installs on macOS, which may lead to an increased number of people who
are looking to try to reinstall Nix without noticing the open/pinned
issue (NixOS#10892) that explains the problem and outlines how to
migrate existing installs.

These admonitions are a short-term measure until we are over the hump
and support volumes dwindle.

src/content/download/02-nix-macos.mdx

@@ -6,6 +6,14 @@ platform: macos
 
 ### Multi-user installation
 
+> **Updating to macOS 15 Sequoia**
+>
+> If you recently updated to macOS 15 Sequoia and are getting
+> ```console
+> error: the user '_nixbld1' in the group 'nixbld' does not exist
+> ```
+> when running Nix commands, refer to GitHub issue [NixOS/nix#10892](https://github.com/NixOS/nix/issues/10892) for instructions to fix your installation without reinstalling.
+
 Install Nix via the **recommended** [multi-user installation](/manual/nix/stable/installation/multi-user):
 
 ```bash