fork.2: NOTES: Descibe clone() call equivalent to fork()

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

man2/fork.2

@@ -205,6 +205,14 @@ wrapper that is provided as part of the
 NPTL threading implementation invokes
 .BR clone (2)
 with flags that provide the same effect as the traditional system call.
+(A call to
+.BR fork ()
+is equivalent to a call to
+.BR clone (2)
+specifying
+.I flags
+as just
+.BR SIGCHLD .)
 The glibc wrapper invokes any fork handlers that have been
 established using
 .BR pthread_atfork (3).