SDL2/SDL_AudioFormat: fixed documentation from header.

SDL2/SDL_AudioFormat.md

@@ -18,6 +18,21 @@ typedef Uint16 SDL_AudioFormat;
 These are what the 16 bits in [SDL_AudioFormat](SDL_AudioFormat) currently
 mean... (Unspecified bits are always zero).
 
+```
+++-----------------------sample is signed if set
+||
+||       ++-----------sample is bigendian if set
+||       ||
+||       ||          ++---sample is float if set
+||       ||          ||
+||       ||          || +---sample bit size---+
+||       ||          || |                     |
+15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
+```
+
+There are macros in SDL 2.0 and later to query these bits.
+
+
 ## Code Examples
 
 ```c++