Merge tag 'doc-4.8-fixes' of git://git.lwn.net/linux

Pull documentation fixes from Jonathan Corbet:
 "Three fixes for the docs build, including removing an annoying warning
  on 'make help' if sphinx isn't present"

* tag 'doc-4.8-fixes' of git://git.lwn.net/linux:
  DocBook: use DOCBOOKS="" to ignore DocBooks instead of IGNORE_DOCBOOKS=1
  Documenation: update cgroup's document path
  Documentation/sphinx: do not warn about missing tools in 'make help'

Documentation/DocBook/Makefile

@@ -6,8 +6,6 @@
 # To add a new book the only step required is to add the book to the
 # list of DOCBOOKS.
 
-ifeq ($(IGNORE_DOCBOOKS),)
-
 DOCBOOKS := z8530book.xml device-drivers.xml \
 	    kernel-hacking.xml kernel-locking.xml deviceiobook.xml \
 	    writing_usb_driver.xml networking.xml \
@@ -19,6 +17,14 @@ DOCBOOKS := z8530book.xml device-drivers.xml \
 	    tracepoint.xml w1.xml \
 	    writing_musb_glue_layer.xml crypto-API.xml iio.xml
 
+ifeq ($(DOCBOOKS),)
+
+# Skip DocBook build if the user explicitly requested no DOCBOOKS.
+.DEFAULT:
+	@echo "  SKIP    DocBook $@ target (DOCBOOKS=\"\" specified)."
+
+else
+
 ###
 # The build process is as follows (targets):
 #              (xmldocs) [by docproc]
@@ -214,16 +220,7 @@ silent_gen_xml = :
 	       -e "s/>/\\>/g";     \
 	   echo "</programlisting>")  > $@
 
-else
-
-htmldocs:
-pdfdocs:
-psdocs:
-xmldocs:
-installmandocs:
-
-endif # IGNORE_DOCBOOKS
-
+endif # DOCBOOKS=""
 
 ###
 # Help targets as used by the top-level makefile
@@ -240,7 +237,7 @@ dochelp:
 	@echo  '  make DOCBOOKS="s1.xml s2.xml" [target] Generate only docs s1.xml s2.xml'
 	@echo  '  valid values for DOCBOOKS are: $(DOCBOOKS)'
 	@echo
-	@echo  "  make IGNORE_DOCBOOKS=1 [target] Don't generate docs from Docbook"
+	@echo  "  make DOCBOOKS=\"\" [target] Don't generate docs from Docbook"
 	@echo  '     This is useful to generate only the ReST docs (Sphinx)'