proc.5: Add reference to core(5) in discussion of 'suid_dumpable'

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

man5/proc.5

@@ -4067,6 +4067,8 @@ or whose binary does not have read permission enabled.
 .TP
 \fI1\ ("debug")\fP
 All processes dump core when possible.
+(Reasons why a process might nevertheless not dump core are described in
+.BR core (5).)
 The core dump is owned by the filesystem user ID of the dumping process
 and no security is applied.
 This is intended for system debugging situations only.