Update documentation for expects

[cozos]

Some pain points I had on learning the doubles API that in my opinion, the documentation could be improved with:

• expects takes precedence over allow (i.e. expects will delete stubs that you created with allow - both previous and subsequent) https://github.com/uber/doubles/blob/master/test/expect_test.py#L89 - maybe another entry to the FAQ section?
• I wanted to stub out a method with predetermined values i.e. allow(user).speak.then_return('blah'), but also wanted to verify that the method was called. Using allow(user).speak.then_return('blah').exactly(2).times sets an upper bound on the stubs, but NOT a lower bound. The docs should indicate that you are able to use then_return on expects

To clarify - reading over the docs you get the impression that the canonical pattern would be to:

# Causes nonintuitive behaviour.
allow(user).speak.then_return('blah')
expect(user).speak.exactly(2).times

Which not only doesn't work, but actually wipes out the stub you created via the first allow. Instead, the docs should show this:

# What we actually wanted.
expect(user).speak.then_return('blah').exactly(2).times