libavutil: Document the shortcommings of av_parse_cpu_flags()

and suggest that av_parse_cpu_caps() be used.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/cpu.h

@@ -75,6 +75,10 @@ attribute_deprecated void av_set_cpu_flags_mask(int mask);
 /**
  * Parse CPU flags from a string.
  *
+ * The returned flags contain the specified flags as well as related unspecified flags.
+ *
+ * This function exists only for compatibility with libav.
+ * Please use av_parse_cpu_caps() when possible.
  * @return a combination of AV_CPU_* flags, negative on error.
  */
 int av_parse_cpu_flags(const char *s);