stdarg.3: Add a note that ... in function signature means a variadic function

Egmont suggested adding this, because the string "..." appears
at several other points in the page, but just to indicate that
some text is omitted from example code.

Reported-by: Egmont Koblinger <egmont@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/stdarg.3

@@ -316,7 +316,8 @@ with each format character based on the type.
 #include <stdarg.h>
 
 void
-foo(char *fmt, ...)
+foo(char *fmt, ...)   /* '...' is C syntax for a variadic function */
+
 {
     va_list ap;
     int d;