Update public FFmpeg header files from r16647 to r18286.

third_party/ffmpeg/include/libavutil/avstring.h

Index: third_party/ffmpeg/include/libavutil/avstring.h
diff --git a/third_party/ffmpeg/include/libavutil/avstring.h b/third_party/ffmpeg/include/libavutil/avstring.h
index 954174555c015ab33e51b1bd4d812a83ccaa9f84..97c2f38715e68673493327706b98aa95797d22bf 100644
--- a/third_party/ffmpeg/include/libavutil/avstring.h
+++ b/third_party/ffmpeg/include/libavutil/avstring.h
@@ -29,7 +29,7 @@
  *
  * @param str input string
  * @param pfx prefix to test
- * @param ptr updated after the prefix in str in there is a match
+ * @param ptr updated if the prefix is matched inside str
  * @return non-zero if the prefix matches, zero otherwise
  */
 int av_strstart(const char *str, const char *pfx, const char **ptr);
@@ -41,14 +41,14 @@ int av_strstart(const char *str, const char *pfx, const char **ptr);
  *
  * @param str input string
  * @param pfx prefix to test
- * @param ptr updated after the prefix in str in there is a match
+ * @param ptr updated if the prefix is matched inside str
  * @return non-zero if the prefix matches, zero otherwise
  */
 int av_stristart(const char *str, const char *pfx, const char **ptr);
 
 /**
  * Copy the string src to dst, but no more than size - 1 bytes, and
- * null terminate dst.
+ * null-terminate dst.
  *
  * This function is the same as BSD strlcpy().
  *
@@ -61,7 +61,7 @@ size_t av_strlcpy(char *dst, const char *src, size_t size);
 
 /**
  * Append the string src to the string dst, but to a total length of
- * no more than size - 1 bytes, and null terminate dst.
+ * no more than size - 1 bytes, and null-terminate dst.
  *
  * This function is similar to BSD strlcat(), but differs when
  * size <= strlen(dst).