Cleanup terminology around fragment/directive

The current spec is somewhat confused about what a "directive" vs "fragment directive" is so make this clearer by:

Make "fragment directive" a concept; it is the part of the URL fragment after :~:. Most existing refs to "fragment directive" now point to this concept rather than the member defined on Document.
Define "directive" as the individual directives inside of the entire "fragment directive"
In light of this, "text fragment directive" doesn't make sense. Rename it to "text directive" which is an existing definition of the struct with parsed values of a text directive.
Rename "text fragment user activation" to "text directive user activation"
Rename the spec from "Text Fragments" to "URL Fragment Text Directive"

Fixes #211

WICG / scroll-to-text-fragment