avcodec/v210: document the requirement for sample_factor

The sample factor must be the same for both 8- and 10-bit functions chosen
otherwise the output will be incorrect.

libavcodec/v210enc.h

@@ -28,7 +28,8 @@ typedef struct V210EncContext {
                         const uint8_t *v, uint8_t *dst, ptrdiff_t width);
     void (*pack_line_10)(const uint16_t *y, const uint16_t *u,
                          const uint16_t *v, uint8_t *dst, ptrdiff_t width);
-    int sample_factor;
+    int sample_factor; /* This value must be the same for both 8-and 10-bit
+                          functions otherwise the output will be incorrect. */
 } V210EncContext;
 
 void ff_v210enc_init(V210EncContext *s);