DocBook: Add a help message for DOCBOOKS env var

Sometimes, it is needed to compile only a subset of the possible
DocBooks. This is supported by the building system, but it is not
docummented. Add a documentation for it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/DocBook/Makefile

@@ -227,6 +227,10 @@ dochelp:
 	@echo  '  mandocs         - man pages'
 	@echo  '  installmandocs  - install man pages generated by mandocs'
 	@echo  '  cleandocs       - clean all generated DocBook files'
+	@echo
+	@echo  'make DOCBOOKS="s1.xml s2.xml" [target] Generate only docs s1.xml s2.xml'
+	@echo  '  valid values for DOCBOOKS are: $(DOCBOOKS)'
+
 
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