adler32: add API documentation

Originally committed as revision 23907 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/adler32.h

@@ -24,6 +24,18 @@
 #include <stdint.h>
 #include "attributes.h"
 
+/**
+ * Calculate the Adler32 checksum of a buffer.
+ *
+ * Passing the return value to a subsequent av_adler32_update() call
+ * allows the checksum of multiple buffers to be calculated as though
+ * they were concatenated.
+ *
+ * @param adler initial checksum value
+ * @param buf   pointer to input buffer
+ * @param len   size of input buffer
+ * @return      updated checksum
+ */
 unsigned long av_adler32_update(unsigned long adler, const uint8_t *buf,
                                 unsigned int len) av_pure;