open.2: NOTES: kcmp() can be used to test if two FDs refer to the same OFD

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

man2/open.2

@@ -1184,6 +1184,13 @@ Each
 of a file creates a new open file description;
 thus, there may be multiple open file descriptions
 corresponding to a file inode.
+
+On Linux, one can use the
+.BR kcmp (2)
+.B KCMP_FILE
+operation to test whether two file descriptors
+(in the same process or in two different processes)
+refer to the same open file description.
 .\"
 .\"
 .SS Synchronized I/O