avio: add detail to avio_printf() size warning

Previous "currently size is limited" didn't give away
much in terms of useful info.
Signed-off-by: Reynaldo H. Verdejo Pinochet <reynaldo@osg.samsung.com>

libavformat/avio.h

@@ -458,7 +458,7 @@ attribute_deprecated
 int url_feof(AVIOContext *s);
 #endif
 
-/** @warning currently size is limited */
+/** @warning Writes up to 4 KiB per call */
 int avio_printf(AVIOContext *s, const char *fmt, ...) av_printf_format(2, 3);
 
 /**

libavformat/aviobuf.c

@@ -968,7 +968,7 @@ int avio_closep(AVIOContext **s)
 int avio_printf(AVIOContext *s, const char *fmt, ...)
 {
     va_list ap;
-    char buf[4096];
+    char buf[4096]; /* update doc entry in avio.h if changed */
     int ret;
 
     va_start(ap, fmt);