Fixed #16856 - Added a way to clear select_related.

Thanks Carl for the suggestion and David Cramer for the patch.

Conflicts:
	docs/releases/1.6.txt
	tests/modeltests/select_related/tests.py

Backport of 5074c75a37f88726f3ae057999144545881d3cfc from master
Fixed #16855 -- select_related() chains as expected.

select_related('foo').select_related('bar') is now equivalent to
select_related('foo', 'bar').

Also reworded docs to recommend select_related(*fields) over select_related()

Conflicts:
	docs/ref/models/querysets.txt
	docs/releases/1.7.txt

Backport of 349c12d3f5c015c2f8b36917da21230c2ac1acb4 from master