scanf.3: Clarify that %n supports type modifiers

My initial reading of this was that type modifiers were probably
not supported.  But they are, and this is actually documented
further up, in the type modifiers documentation.  But to make it
clearer, let's copy the language that printf(3) has in its %n
section.

Signed-off-by: Alyssa Ross <hi@alyssa.is>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/scanf.3

@@ -526,7 +526,9 @@ the next pointer must be a pointer to a pointer to
 Nothing is expected; instead, the number of characters consumed thus far
 from the input is stored through the next pointer, which must be a pointer
 to
-.IR int .
+.IR int ,
+or variant whose size matches the (optionally)
+supplied integer length modifier.
 This is
 .I not
 a conversion and does