Add a brief comment explaining why the overload is needed.

include/fmt/core.h

@@ -508,6 +508,10 @@ namespace internal {
 
 struct dummy_string_view { typedef void char_type; };
 dummy_string_view to_string_view(...);
+// The following overload disables using std::FILE as a character type, which is
+// needed because or else
+//   fmt::print(stderr, fmt::emphasis::bold, "");
+// would take stderr as the format string.
 dummy_string_view to_string_view(const std::FILE *);
 using fmt::v5::to_string_view;