docs: Rebuild docs from scratch, as required for proper only:: handling.

Docs are now by default rebuilt from scratch, as required to build
conditionalized (i.e. using only:: directive) docs across different
output types. We have pretty small docset, so that's still rather fast.
However, if that's a concern, incremental rebuilds can be used by
passing "FORCE=" (nothing after =) as a make parameter. This will work
when using the same output type (e.g. only "html").

docs/Makefile

@@ -6,6 +6,9 @@ SPHINXOPTS    =
 SPHINXBUILD   = sphinx-build
 PAPER         =
 BUILDDIR      = build/$(MICROPY_PORT)
+# Run "make FORCE= ..." to avoid rebuilding from scratch (and risk
+# producing incorrect docs).
+FORCE         = -E
 
 # User-friendly check for sphinx-build
 ifeq ($(shell which $(SPHINXBUILD) >/dev/null 2>&1; echo $$?), 1)
@@ -50,7 +53,7 @@ clean:
 	rm -rf $(BUILDDIR)/*
 
 html:
-	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
+	$(SPHINXBUILD) $(FORCE) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."
 
@@ -111,7 +114,7 @@ latex:
 	      "(use \`make latexpdf' here to do that automatically)."
 
 latexpdf:
-	$(SPHINXBUILD) -b latex $(ALLSPHINXOPTS) $(BUILDDIR)/latex
+	$(SPHINXBUILD) $(FORCE) -b latex $(ALLSPHINXOPTS) $(BUILDDIR)/latex
 	@echo "Running LaTeX files through pdflatex..."
 	$(MAKE) -C $(BUILDDIR)/latex all-pdf
 	@echo "pdflatex finished; the PDF files are in $(BUILDDIR)/latex."