POWER: Feedback.

This patch fiddles with the example spec text to clarify the intent of
the placeholder text in step 4, and adjusts the discussion of
identifiers in order to make the categorizations clear.

This is in response to the suggestions in [1] and [2].

[1]: http://lists.w3.org/Archives/Public/public-webappsec/2014Dec/0029.html
[2]: http://lists.w3.org/Archives/Public/public-webappsec/2014Dec/0030.html