Rollup of 3 pull requests
Successful merges:
- #113253 (Fixed documentation of from<CString> for Rc<CStr>: Arc -> Rc)
- #113258 (Migrate GUI colors test to original CSS color format)
- #113259 (Suggest `x build library` for a custom toolchain that fails to load `core`)
r? `@ghost`
`@rustbot` modify labels: rollup