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>

Documentation/kernel-doc-nano-HOWTO.txt

@@ -142,9 +142,10 @@ are:
 
 - Makefile
 
-  The targets 'sgmldocs', 'psdocs', 'pdfdocs', and 'htmldocs' are used
-  to build DocBook files, PostScript files, PDF files, and html files
-  in Documentation/DocBook.
+  The targets 'xmldocs', 'psdocs', 'pdfdocs', and 'htmldocs' are used
+  to build XML DocBook files, PostScript files, PDF files, and html files
+  in Documentation/DocBook. The older target 'sgmldocs' is equivalent
+  to 'xmldocs'.
 
 - Documentation/DocBook/Makefile
 
@@ -158,8 +159,8 @@ If you just want to read the ready-made books on the various
 subsystems (see Documentation/DocBook/*.tmpl), just type 'make
 psdocs', or 'make pdfdocs', or 'make htmldocs', depending on your
 preference.  If you would rather read a different format, you can type
-'make sgmldocs' and then use DocBook tools to convert
-Documentation/DocBook/*.sgml to a format of your choice (for example,
+'make xmldocs' and then use DocBook tools to convert
+Documentation/DocBook/*.xml to a format of your choice (for example,
 'db2html ...' if 'make htmldocs' was not defined).
 
 If you want to see man pages instead, you can do this: