POWER: Cleaning up definitions.

In response to Chaals' [1], this patch cleans up the definitions section
of the specification. This breaks down into three changes:

1.  Flavor text has been dropped from section 2.1; it now simply lists
    defined terms, and points to their definitions elsewhere in the
    document.

2.  The <dfn> tags have been removed from section 2.2, ensuring that
    Bikeshed's glorious autolinks for those terms point outside this
    document.

3.  "Environment settings object" is now "settings object", which is the
    most that W3C and WHATWG's respective documents seem to be able to
    agree upon.

[1]: http://lists.w3.org/Archives/Public/public-webappsec/2014Nov/0360.html