Document buffer format for audio processors.

[AlbertoGP]

There is a comment in the example

raylib/examples/audio/audio_mixed_processor.c

Line 26 in d6f16b7

float *samples = (float *)buffer; // Samples internally stored as <float>s

about this, but it is more discoverable in the API docs.

[AlbertoGP]

From the stream effects example

raylib/examples/audio/audio_stream_effects.c

Line 154 in d6f16b7

float l = ((float *)buffer)[i], r = ((float *)buffer)[i + 1];

it seems that this applies to AttachAudioStreamProcessor() too, should I add the note also there?

[raysan5]

@AlbertoGP thanks for the review!