Update StandardCodec documentation with double alignment (#58456)

packages/flutter/lib/src/services/message_codecs.dart

@@ -243,7 +243,8 @@ class StandardMessageCodec implements MessageCodec<dynamic> {
   // * Larger integers are encoded using 8 bytes two's complement
   //   representation.
   // * doubles are encoded using the IEEE 754 64-bit double-precision binary
-  //   format.
+  //   format. Zero bytes are added before the encoded double value to align it
+  //   to a 64 bit boundary in the full message.
   // * Strings are encoded using their UTF-8 representation. First the length
   //   of that in bytes is encoded using the expanding format, then follows the
   //   UTF-8 encoding itself.