Language & number consistency

[toddbaert]

This spec has 2 commits, I strongly suggest you look at them one at a time, since the second is purely whitespace changes.

1st commit:

• breaks the flag-evaluation.md doc into sections, more like the hooks.md doc, to prevent so much number thrashing in the future
• improves consistency of terminology across files (see comments)
• specifies that both error and finally user-defined hooks may throw. This may be controversial, but I think it makes sense (and finally can already throw, as per spec). User-defined code being able to throw seems like a plus too me. See comments.

2nd commit:

• sets proseWrap: never in prettier. This improves readability in PRs. As it stands, the fact that line-wrapping is on adds a lot of unneeded deltas in commits, since adding a new word might totally change the way the wrapping ends up. If we keep everything to a single line, it makes diffing much easier. This is mentioned in the style guide

Fixes open-feature/spec#38

[toddbaert]

@justinabrahms I think I've addressed all your concerns except the big one, which I personally feel unsure about: https://github.com/open-feature/spec/pull/90#discussion_r886087152