P1673: Should sqrt be a customization point?

@crtrott This issue addresses the remaining P1673 points in #281.

1. Specify whether we are using std::sqrt (or ::sqrt) (important, because it has user-visible side effects like errno).
2. Is sqrt a customization point, like abs and conj are? If so, add it to front matter.

The second point asks whether P1673 should make sqrt a customization point, just like conj-if-needed. sqrt has the problem that it turns integral arguments into double. Thus, it has type preservation issues analogous to conj. However, unlike conj, we can't trivially fix sqrt for integral arguments.

Start by reading the remaining P1673 points referenced from #281 and the surrounding proposal text. Resolve whether sqrt uses std::sqrt or ::sqrt and whether it is a customization point; done means the decision is documented and the relevant front matter is updated if needed.