Add a comment about clang

Add a comment that clang now doesn't allow dllexport on template instantiation definitions.
Mark them on tempalte instantiation declarations instead (in format.h).

[MinGW] Fix dllexport of explicit template instantiation
https://reviews.llvm.org/D61118

src/format.cc

@@ -55,6 +55,7 @@ vformat_to(buffer<char>&, string_view,
                type_identity_t<char>>>);
 }  // namespace detail
 
+// Clang doesn't allow dllexport on template instantiation definitions (LLVM D61118)
 template struct detail::basic_data<void>;
 
 // Workaround a bug in MSVC2013 that prevents instantiation of format_float.