swresample: add SwrContext doxy

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswresample/swresample.h

@@ -149,6 +149,12 @@ enum SwrFilterType {
  * @}
  */
 
+/**
+ * The libswresample context. Unlike libavcodec and libavformat, this structure
+ * is opaque. This means that if you would like to set options, you must use
+ * the @ref avoptions API and cannot directly set values to members of the
+ * structure.
+ */
 typedef struct SwrContext SwrContext;
 
 /**