lavu/base64: extend/clarify doxy for the base64 API

Also improve overall consistency.

libavutil/base64.h

@@ -46,15 +46,17 @@ int av_base64_decode(uint8_t *out, const char *in, int out_size);
  * Encode data to base64 and null-terminate.
  *
  * @param out      buffer for encoded data
- * @param out_size size in bytes of the output buffer, must be at
- *                 least AV_BASE64_SIZE(in_size)
- * @param in_size  size in bytes of the 'in' buffer
- * @return         'out' or NULL in case of error
+ * @param out_size size in bytes of the out buffer (including the
+ *                 null terminator), must be at least AV_BASE64_SIZE(in_size)
+ * @param in       input buffer containing the data to encode
+ * @param in_size  size in bytes of the in buffer
+ * @return         out or NULL in case of error
  */
 char *av_base64_encode(char *out, int out_size, const uint8_t *in, int in_size);
 
 /**
- * Calculate the output size needed to base64-encode x bytes.
+ * Calculate the output size needed to base64-encode x bytes to a
+ * null-terminated string.
  */
 #define AV_BASE64_SIZE(x)  (((x)+2) / 3 * 4 + 1)