kernel-doc: Update references to SGML to refs to XML instead.

Since the "sgmldocs" target is simply a pointer to the newer "xmldocs"
target, update the documentation to reflect this.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>