Improve docs

Author: aibaars

rust/ql/.generated.list

@@ -179,7 +179,7 @@ class Function(Declaration):
 @rust.doc_test_signature("() -> ()")
 class MissingExpr(Expr):
     """
-    A missing expression, used as a place holder for incomplete syntax, as well as bodies of functions that are defined externally.
+    A missing expression, used as a placeholder for incomplete syntax.
 
     ```
     let x = non_existing_macro!();