Add a Panics section to component_unavailable_msg's docstring

Author: rami3l

src/errors.rs

@@ -170,6 +170,10 @@ fn remove_component_msg(cs: &Component, manifest: &Manifest, toolchain: &str) ->
     }
 }
 
+/// Returns a error message indicating that certain [`Component`]s are unavailable.
+///
+/// # Panics
+/// This function will panic when the collection of unavailable components `cs` is empty.
 fn component_unavailable_msg(cs: &[Component], manifest: &Manifest, toolchain: &str) -> String {
     let mut buf = vec![];
     match cs {