Include NullSink in std.range ddocs.

Partial fix for makefile to build std.range docs, that was broken after
std.range was split. This is only a partial fix; the full fix will have
to be done as part of [issue 13766] (https://issues.dlang.org/show_bug.cgi?id=13766).
Merge pull request #2763 from quickfur/nullsink_ddoc

Include NullSink in std.range ddocs.