improve <charconv>

Author: suomesta

reference/charconv.md

@@ -66,7 +66,7 @@ XMLやJSONに代表されるテキストベースのデータを利用するケ
 - C++17
 
 ### 処理系
-- [Clang](/implementation.md#clang): 7.0(整数のみ) [mark verified], 14.0(to_chars()のみ浮動小数点もsupport) [mark verified]
+- [Clang](/implementation.md#clang): 7.0(整数のみ) [mark verified], 14.0(to_chars()のみ浮動小数点もsupport), 20.1(from_chars()もdouble, floatをsupport) [mark verified]
 - [GCC](/implementation.md#gcc): 8.0(整数のみ) [mark verified], 11.1(full support) [mark verified]
 - [Visual C++](/implementation.md#visual_cpp): 2017 update 7(整数のみ) [mark verified], update 9(full support) [mark verified]
 

reference/charconv/from_chars.md

@@ -514,7 +514,7 @@ int main() {
 - C++17
 
 ### 処理系
-- [Clang](/implementation.md#clang): 7.0(整数のみ) [mark verified]
+- [Clang](/implementation.md#clang): 7.0(整数のみ), 20.1(from_chars()もdouble, floatをsupport) [mark verified]
 - [GCC](/implementation.md#gcc): 8.0(整数のみ) [mark verified], 11.1(full support) [mark verified]
 - [Visual C++](/implementation.md#visual_cpp): 2017 update 7(整数のみ) [mark verified], update 9(full support) [mark verified]
 

reference/charconv/to_chars.md

@@ -396,10 +396,12 @@ z
 - C++17
 
 ### 処理系
-- [Clang](/implementation.md#clang): 7.0(整数のみ) [mark verified], 14.0(full support) [mark verified]
+- [Clang](/implementation.md#clang): 7.0(整数のみ) [mark verified], 14.0(full support ※) [mark verified]
 - [GCC](/implementation.md#gcc): 8.0(整数のみ) [mark verified], 11.1(full support) [mark verified]
 - [Visual C++](/implementation.md#visual_cpp): 2017 update 7(整数のみ) [mark verified], update 9(full support) [mark verified]
 
+※ Clangの実装では、(8), (12)は第３引数のlong double変数が関数内でdouble型に変換される。
+
 ## 関連項目
 - [`chars_format`](../charconv/chars_format.md)
 - [`to_chars_result`](../charconv/to_chars_result.md)