ldconfig.8: Document -c/--format option

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

man8/ldconfig.8

@@ -99,6 +99,15 @@ should normally be run by the superuser as it may require write
 permission on some root owned directories and files.
 .SH OPTIONS
 .TP
+.BR \-c " \fIfmt\fP, " \-\-format=\fIfmt\fP
+(Since glibc 2.2)
+Cache format to use:
+.IR old ,
+.IR new ,
+or
+.IR compat
+(default).
+.TP
 .B \-C cache
 Use
 .B cache