Simplify allowing warnings during ARM build

RHEL8 Sphinx does not support all features used in ARM building. But
with few emitted warnings it can build the documentation fine. Simplify
warnings acceptance by allowing make doc SPHINX_W=''.

Makefile.docs

@@ -2,6 +2,7 @@ SPHINX_V = $(SPHINX_V_@AM_V@)
 SPHINX_V_ = $(SPHINX_V_@AM_DEFAULT_V@)
 SPHINX_V_0 = -q
 SPHINX_V_1 = -n
+SPHINX_W = -W
 
 AM_V_SPHINX = $(AM_V_SPHINX_@AM_V@)
 AM_V_SPHINX_ = $(AM_V_SPHINX_@AM_DEFAULT_V@)
@@ -20,7 +21,7 @@ SESSION_KEY = .. |session_key| replace:: ``$(runstatedir)/session.key``
 export RST_EPILOG = $(RNDC_CONF)$(LF)$(RNDC_KEY)$(LF)$(NAMED_CONF)$(LF)$(BIND_KEYS)$(LF)$(NAMED_PID)$(LF)$(SESSION_KEY)
 
 common_SPHINXOPTS =			\
-	-W				\
+	$(SPHINX_W)			\
 	-c $(srcdir)			\
 	-a				\
 	$(SPHINX_V)