Improve comments and fix invariant message

that was not up-to-date.

Author: petr-bauch

src/goto-programs/remove_function_pointers.cpp

@@ -150,9 +150,12 @@ class remove_function_pointerst
   /// if (fp=&f1) then goto loc1
   /// if (fp=&f2) then goto loc2
   /// ..
-  /// loc1: f1(); goto N+1;
-  /// loc2: f2(); goto N+1;
+  /// if (fp=&fn) then goto locN
+  /// loc1:  f1(); goto N+1;
+  /// loc2:  f2(); goto N+1;
   /// ..
+  /// locN:  fn(); goto N+1;
+  /// locN+1:
   ///
   /// \param functions: set of function candidates
   /// \param code: the original function call

src/goto-programs/remove_returns.cpp

@@ -196,8 +196,8 @@ bool remove_returnst::do_function_calls(
     INVARIANT(
       function_call.function().id() == ID_symbol ||
         function_call.function().id() == ID_dereference,
-      "indirect function calls should have been removed prior to running "
-      "remove-returns");
+      "only direct function calls (via a symbol) or indirect function calls "
+      "(via a dereference) are supported");
 
     bool is_function_pointer = function_call.function().id() == ID_dereference;