[DOC] Comment about logic to care about static/dynamic hetero build

The condition is rarely met in usual cases, so it's worth to note about
the situation and the reason why we care about it.

template/exts.mk.tmpl

@@ -69,6 +69,13 @@ Dir.glob("{ext,.bundle/gems}/*/exts.mk") do |e|
   end
 end
 deps.uniq!
+
+# NOTE: Only if extensions are configured as static and dynamic heterogeneously
+# (e.g. --with-static-linked-ext=foo or ext/Setup can mix static and dynamic
+# extensions), EXTOBJS may contain both extinit.o and dmyext.o. In such case,
+# prefer extinit.o, which does actual Init_${ext} function calls for statically
+# linked extensions, and drop dmyext.o, which does nothing but just to make the
+# linker happy.
 if objs = macros["EXTOBJS"] and objs.any? {|e|e.start_with?("ext/extinit.")}
   objs.delete_if {|e|e.start_with?("dmyext.")}
 end