audio: clarified what SDL_AudioSpec::samples is, removed note about power of 2.

These don't have to be power-of-2 sizes anymore because of SDL_AudioStream,
and the new resampler, but also, many platforms don't give you power-of-2 DMA
buffer in the first place!

include/SDL_audio.h

@@ -171,7 +171,7 @@ typedef struct SDL_AudioSpec
     SDL_AudioFormat format;     /**< Audio data format */
     Uint8 channels;             /**< Number of channels: 1 mono, 2 stereo */
     Uint8 silence;              /**< Audio buffer silence value (calculated) */
-    Uint16 samples;             /**< Audio buffer size in samples (power of 2) */
+    Uint16 samples;             /**< Audio buffer size in sample FRAMES (total samples divided by channel count) */
     Uint16 padding;             /**< Necessary for some compile environments */
     Uint32 size;                /**< Audio buffer size in bytes (calculated) */
     SDL_AudioCallback callback; /**< Callback that feeds the audio device (NULL to use SDL_QueueAudio()). */