random_seed: more elaborate doxy.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/random_seed.h

@@ -25,6 +25,10 @@
 
 /**
  * Get a seed to use in conjunction with random functions.
+ * This function tries to provide a good seed at a best effort bases.
+ * Its possible to call this function multiple times if more bits are needed.
+ * It can be quite slow, which is why it should only be used as seed for a faster
+ * PRNG. The quality of the seed depends on the platform.
  */
 uint32_t av_get_random_seed(void);