Documentation: add top level 'make help' output for Sphinx

While there's slight overlap with the DocBook help now, this can stay
intact when the DocBook help goes away.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>

Documentation/DocBook/Makefile

@@ -218,7 +218,7 @@ silent_gen_xml = :
 ###
 # Help targets as used by the top-level makefile
 dochelp:
-	@echo  ' Linux kernel internal documentation in different formats:'
+	@echo  ' Linux kernel internal documentation in different formats (DocBook):'
 	@echo  '  htmldocs        - HTML'
 	@echo  '  pdfdocs         - PDF'
 	@echo  '  psdocs          - Postscript'

Documentation/Makefile.sphinx

@@ -60,4 +60,12 @@ installmandocs:
 cleandocs:
 	$(Q)rm -rf $(BUILDDIR)
 
+dochelp:
+	@echo  ' Linux kernel internal documentation in different formats (Sphinx):'
+	@echo  '  htmldocs        - HTML'
+	@echo  '  pdfdocs         - PDF'
+	@echo  '  epubdocs        - EPUB'
+	@echo  '  xmldocs         - XML'
+	@echo  '  cleandocs       - clean all generated files'
+
 endif # HAVE_SPHINX

Makefile

@@ -1364,6 +1364,8 @@ help:
 	@$(MAKE) $(build)=$(package-dir) help
 	@echo  ''
 	@echo  'Documentation targets:'
+	@$(MAKE) -f $(srctree)/Documentation/Makefile.sphinx dochelp
+	@echo  ''
 	@$(MAKE) -f $(srctree)/Documentation/DocBook/Makefile dochelp
 	@echo  ''
 	@echo  'Architecture specific targets ($(SRCARCH)):'