Update assert and adjust docs

src/lib.rs

@@ -611,8 +611,7 @@ impl Config {
     /// This is only relevant for C and C++ files.
     ///
     /// # Panics
-    /// Panics if more than one file is present in the config, as macro
-    /// expansion only makes sense for a single file.
+    /// Panics if more than one file is present in the config.
     ///
     /// # Example
     /// ```no_run
@@ -628,7 +627,7 @@ impl Config {
         }
         cmd.arg(compiler.family.expand_flag());
 
-        assert_eq!(self.files.len(), 1,
+        assert!(self.files.len() <= 1,
                 "Expand may only be called for a single file");
 
         for file in self.files.iter() {