docs(dev-desktops): document available disk space

src/infra/docs/dev-desktop.md

@@ -186,6 +186,23 @@ It helps to check <https://packages.ubuntu.com/> first and make sure that the
 package is available for both `arm64` and `amd64` architectures. The dev
 desktops are currently running Ubuntu 22.04 LTS.
 
+## Available disk space
+
+Every user has a quota of how much disk space they can use.
+Currently, the quota is set to
+[150GB](https://github.com/rust-lang/simpleinfra/blob/8a59e4faeb75a09b072671c74a7cb70160ebef50/ansible/roles/dev-desktop/defaults/main.yml#L7).
+
+Exceeding the quota will result in the error `Disk quota exceeded (os error 122)`.
+
+To save disk space:
+
+- Remove your `build/` directory to clear outdated artifacts.
+- Remove `build-rust-analyzer/` (or whatever it's named as) if you have a
+  separate rust-analyzer build directory.
+- Remove unneeded toolchains with `rustup toolchain uninstall`.
+  You can list the installed toolchains with `rustup toolchain list`.
+  `cargo-bisect-rustc` or using `--preserve` explicitly may leave nightly toolchains behind.
+
 ## How to give feedback and report issues
 
 If you experience any problems with the dev desktops, or have feedback and