Change RFC so that only ops are deterministic

The RFC was changed so that only individual ops are deterministic, not TensorFlow as a whole. This limits the scope of the API, while still allowing users to write deterministic models.

Additional changes:
  * Add "Testing plan" section
  * Add original RFC's API to "Alternatives considered"
  * Add more ops to "Op Review and changes" section
  * Raise FailedPreconditionError when op is fundamentally not deterministic, not a NotImplementedError
  * Remove discussion section, since it doesn't apply to the new version of the RFC. There is a link to the old version of the RFC (with the discussion section) in "Alternatives considered"
  * Alphabetize author list
  * Move some info from "Objective" to "Design proposal"
  * Grammar fixes

Author: reedwm