update selectin docs

update selectin docs

* correct many-to-one example that doesnt use JOIN or ORDER BY
anymore

* Oracle does tuple IN, let's test it

* many-to-many is supported but joins all the way right now

* remove verbiage about yield_per for the moment to simplify
updates to how yield_per works w/ new style execution.  yield_per
is difficult to explain and the section seems kind of complicated
with those details added at the moment.

Change-Id: I010ed36f554f06310f336a5b12760c447b38ec01
(cherry picked from commit 72fa0ece049f08d5d24940272b80b1290fef6192)