add docbook version macro of @sub

git-svn-id: svn://svn.h5l.se/heimdal/trunk/heimdal@14910 ec53bebd-3082-4978-b11e-865c3cabbd6b

doc/whatis.texi

@@ -46,6 +46,14 @@ services can authenticate each other.
 @end macro
 @end ifhtml
 
+@ifdocbook
+@macro sub{arg}
+@docbook
+<subscript>\arg\</subscript>
+@end docbook
+@end macro
+@end ifdocbook
+
 @quotation
 @strong{Note} This discussion is about Kerberos version 4, but version
 5 works similarly.