doc: Revert part of 0878a568f9 for Texinfo 5.1

0878a568f9 fixed a warning from Texinfo 6.7:

    doc/whatis.texi:33: warning: redefining Texinfo language command: @sub

but the fix makes the doc build fail w/ Texinfo 5.1.

There may still be other problems when using Texinfo 5.1.

doc/whatis.texi

@@ -29,6 +29,28 @@ services can authenticate each other.
 
 @section Basic mechanism
 
+@ifinfo
+@macro sub{arg}
+<\arg\>
+@end macro
+@end ifinfo
+
+@iftex
+@macro sub{arg}
+@textsubscript{\arg\}
+@end macro
+@end iftex
+
+@ifhtml
+@macro sub{arg}
+
+@html
+<sub>\arg\</sub>
+@end html
+
+@end macro
+@end ifhtml
+
 @c ifdocbook
 @c macro sub{arg}
 @c docbook