add_key.2: Document "big_key" key type

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

man2/add_key.2

@@ -117,6 +117,16 @@ to ensure that it is qualified by a "service" prefix,
 by checking to ensure that the
 .I description
 contains a ':' that is preceded by other characters.
+.TP
+.IR """big_key""" " (since Linux 3.13)"
+.\" commit ab3c3587f8cda9083209a61dbe3a4407d3cada10
+This key type is similar to
+.IR """user""" ,
+but may hold a payload of up to 1 MiB.
+For this key type,
+the contents can be saved in a tmpfs filesystem and
+thus swapped out to disk when memory pressure is high.
+This key type is useful for tasks such as holding Kerberos ticket caches.
 .SH RETURN VALUE
 On success,
 .BR add_key ()