Add warnings to av_strlcat and av_strlcpy doxygen that the input strings

already must be properly 0-terminated strings. Unlike strncpy it
cannot be used on raw data read from a file.

Originally committed as revision 19800 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/avstring.h

@@ -56,6 +56,10 @@ int av_stristart(const char *str, const char *pfx, const char **ptr);
  * @param src source string
  * @param size size of destination buffer
  * @return the length of src
+ *
+ * WARNING: since the return value is the length of src, src absolutely
+ * _must_ be a properly 0-terminated string, otherwise this will read beyond
+ * the end of the buffer and possibly crash.
  */
 size_t av_strlcpy(char *dst, const char *src, size_t size);
 
@@ -70,6 +74,10 @@ size_t av_strlcpy(char *dst, const char *src, size_t size);
  * @param src source string
  * @param size size of destination buffer
  * @return the total length of src and dst
+ *
+ * WARNING: since the return value use the length of src and dst, these absolutely
+ * _must_ be a properly 0-terminated strings, otherwise this will read beyond
+ * the end of the buffer and possibly crash.
  */
 size_t av_strlcat(char *dst, const char *src, size_t size);