Clarify use of the "today" Sphinx variable

Add a comment explaining the purpose of setting the "today" variable in
Sphinx invocations to prevent confusion caused by the absence of that
variable from reStructuredText sources.

Drop the -A command-line option from the sphinx-build invocation for
EPUB output as "today" is already set in the ALLSPHINXOPTS variable.

Makefile.docs

@@ -15,6 +15,8 @@ common_SPHINXOPTS =			\
 	-a				\
 	$(SPHINX_V)
 
+# The "today" variable set below is not directly used in the ARM, but its value
+# is implicitly inserted on the title page of the PDF file produced by Sphinx.
 ALLSPHINXOPTS =				\
 	$(common_SPHINXOPTS)		\
 	-D today="$(RELEASE_DATE)"	\

doc/arm/Makefile.am

@@ -65,7 +65,7 @@ install-singlehtml: singlehtml
 	cp -R $(SPHINXBUILDDIR)/singlehtml/_static/* $(DESTDIR)/$(docdir)/_static/
 
 epub:
-	$(AM_V_SPHINX)$(SPHINX_BUILD) -b epub -A today=$(RELEASE_DATE) -d $(SPHINXBUILDDIR)/.doctrees/epub $(ALLSPHINXOPTS) $(SPHINXBUILDDIR)/epub
+	$(AM_V_SPHINX)$(SPHINX_BUILD) -b epub -d $(SPHINXBUILDDIR)/.doctrees/epub $(ALLSPHINXOPTS) $(SPHINXBUILDDIR)/epub
 
 install-epub:
 	$(INSTALL) -d $(DESTDIR)/$(docdir)