docs: note std::format hex float 0x prefix difference

Document that fmt's 'a'/'A' presentation adds a 0x/0X prefix while
std::format (via std::to_chars) does not. Closes #4657.

doc/api.md

@@ -715,6 +715,10 @@ following differences:
   (ignoring redundant digits and sign in exponent) and may produce more
   decimal digits than necessary.
 
+- For the hexadecimal floating-point presentation types (`'a'`/`'A'`),
+  {fmt} includes a `0x`/`0X` prefix (similar to `printf`'s `%a`), while
+  `std::format` follows `std::to_chars` and omits the prefix.
+
 ## Configuration Options
 
 {fmt} provides configuration via CMake options and preprocessor macros to