avformat/avio: fix avio_feof documentation

It has been this way too long to change behaviour, so let's change the docs
instead.
Signed-off-by: NMarton Balint <cus@passwd.hu>

libavformat/avio.h

@@ -236,7 +236,7 @@ typedef struct AVIOContext {
     int (*write_packet)(void *opaque, uint8_t *buf, int buf_size);
     int64_t (*seek)(void *opaque, int64_t offset, int whence);
     int64_t pos;            /**< position in the file of the current buffer */
-    int eof_reached;        /**< true if eof reached */
+    int eof_reached;        /**< true if was unable to read due to error or eof */
     int write_flag;         /**< true if open for writing */
     int max_packet_size;
     unsigned long checksum;
@@ -566,8 +566,8 @@ static av_always_inline int64_t avio_tell(AVIOContext *s)
 int64_t avio_size(AVIOContext *s);
 
 /**
- * feof() equivalent for AVIOContext.
- * @return non zero if and only if end of file
+ * Similar to feof() but also returns nonzero on read errors.
+ * @return non zero if and only if at end of file or a read error happened when reading.
  */
 int avio_feof(AVIOContext *s);