Collapses 2.13 and 2.14 into one and changes the wording of the rule to ...

...be positive.

Also includes minor reformatting.

Also reformulates 1.9 in the same wording as 2.13

As discussed in #106, with positive rule focus suggested by @DougLea

Review by @reactive-streams/contributors 
After feedback from @DougLea, what about:

+| <a name="1.8">8</a> | Upon cancellation of a `Subscription`, its associated `Subscriber` MUST eventually stop being sent signals. |