Document the av_base64_encode/decode functions.

See the thread:
"[PATCH] Improve documentation for libavutil/base64.h".

Originally committed as revision 17074 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/base64.h

@@ -24,15 +24,25 @@
 #include <stdint.h>
 
 /**
- * Decodes Base64.
- * Parameter order is the same as strncpy().
+ * Decodes the base64-encoded string in \p in and puts the decoded
+ * data in \p out.
+ *
+ * @param out_size size in bytes of the \p out buffer, it should be at
+ * least 3/4 of the length of \p in
+ * @return the number of bytes written, or a negative value in case of
+ * error
  */
 int av_base64_decode(uint8_t *out, const char *in, int out_size);
 
 /**
- * Encodes Base64.
- * @param src data, not a string
- * @param buf output string
+ * Encodes in base64 the data in \p in and puts the resulting string
+ * in \p out.
+ *
+ * @param out_size size in bytes of the \p out string, it should be at
+ * least ((\p in_size + 2) / 3) * 4 + 1
+ * @param in_size size in bytes of the \p in buffer
+ * @return the string containing the encoded data, or NULL in case of
+ * error
  */
 char *av_base64_encode(char *out, int out_size, const uint8_t *in, int in_size);