epoll_create.2: Say more about unused epoll_create() 'size' arg

Supply a little more explanation about why the 'size' argument
of epoll_create() is nowadays ignored.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/epoll_create.2

@@ -108,6 +108,8 @@ is Linux-specific, and was introduced in kernel 2.5.44.
 Since Linux 2.6.8, the
 .I size
 argument is unused.
+(The kernel dynamically sizes the required data structures
+without needing this initial hint.)
 .SH "SEE ALSO"
 .BR close (2),
 .BR epoll_ctl (2),