libstdc++: Fix comment on std::print helper __open_terminal

The comment describes an earlier version of the function which I
experimented with when implementing the std::print feature. Update it to
describe the current semantics.

libstdc++-v3/ChangeLog:

	* src/c++23/print.cc (__open_terminal): Fix comment.

libstdc++-v3/src/c++23/print.cc

@@ -75,8 +75,9 @@ namespace
 } // namespace
 #endif
 
-  // This returns intptr_t that is either a Windows HANDLE
-  // or 1 + a POSIX file descriptor. A zero return indicates failure.
+  // Windows: If `f` refers to a console, return the HANDLE associated with it.
+  // POSIX: If `f` refers to a POSIX terminal, return `f`.
+  // Return a null pointer if `f` is not known to be a console or terminal.
   void*
   __open_terminal([[maybe_unused]] FILE* f)
   {
@@ -95,6 +96,7 @@ namespace
     return nullptr;
   }
 
+  // If a `FILE*` can be obtained from `sb`, behave as above.
   void*
   __open_terminal([[maybe_unused]] std::streambuf* sb)
   {