doc-rst: Fix recursive make invocation from macros

In any case where we recurse but don't mention $(MAKE) literally in
the recipe, we need to add a '+' at the start of the command to ensure
that parallel makes and various other options work properly.

Fixes: 609afe6b49ef ("Documentation/sphinx: build the media intermediate ...")
Tested-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/Makefile.sphinx

@@ -66,10 +66,10 @@ quiet_cmd_sphinx = SPHINX  $@ --> file://$(abspath $(BUILDDIR)/$3/$4)
 	$(abspath $(BUILDDIR)/$3/$4)
 
 htmldocs:
-	@$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,html,$(var),,$(var)))
+	@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,html,$(var),,$(var)))
 
 latexdocs:
-	@$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,latex,$(var),latex,$(var)))
+	@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,latex,$(var),latex,$(var)))
 
 ifeq ($(HAVE_PDFLATEX),0)
 
@@ -85,10 +85,10 @@ pdfdocs: latexdocs
 endif # HAVE_PDFLATEX
 
 epubdocs:
-	@$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,epub,$(var),epub,$(var)))
+	@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,epub,$(var),epub,$(var)))
 
 xmldocs:
-	@$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,xml,$(var),xml,$(var)))
+	@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,xml,$(var),xml,$(var)))
 
 # no-ops for the Sphinx toolchain
 sgmldocs: