Add a comment about a VC6 limitation.

miniaudio.h

@@ -1406,6 +1406,7 @@ Some backends have some nuance details you may want to be aware of.
 - The sndio backend is currently only enabled on OpenBSD builds.
 - The audio(4) backend is supported on OpenBSD, but you may need to disable sndiod before you can use it.
 - Note that GCC and Clang requires `-msse2`, `-mavx2`, etc. for SIMD optimizations.
+- When compiling with VC6 and earlier, decoding is restricted to files less than 2GB in size. This is due to 64-bit file APIs not being available.
 */
 
 #ifndef miniaudio_h