cpu: improve av_get_cpu_flags() doxy

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

libavutil/cpu.h

@@ -60,6 +60,9 @@
 
 /**
  * Return the flags which specify extensions supported by the CPU.
+ * The returned value is affected by av_force_cpu_flags() if that was used
+ * before. So av_get_cpu_flags() can easily be used in a application to
+ * detect the enabled cpu flags.
  */
 int av_get_cpu_flags(void);