Mark single-implementation features "at risk" instead of distinguishing the CR-draft stage

[jyasskin]

In drafting the PATWG charter, I suggested the language that says

The WG will progress its normative specifications through the following standardization process: First Public Working Draft, Working Draft, Candidate Recommendation Draft, and Candidate Recommendation Snapshot. It is expected that to reach the Candidate Recommendation Snapshot stage, each normative specification is expected to have at least two independent implementations of every feature defined in the specification.

in order to provide separate stages for

1. CR Draft: a place the WG can declare that some features were ready to implement without requiring them to have already been implemented twice, and
2. CR Snapshot: a place the WG can declare that features have been interoperably implemented.

@tantek pointed out in his charter review that the Process doesn't really endorse this separation, and separately he pointed out that the CSSWG marks features "at risk" when they're in the state I used CR Drafts to accommodate. I suggest we solve the Process problem by replacing the above language with something like

... It is expected that in Candidate Recommendations, every feature that doesn't have at least two independent implementations will be clearly marked "at risk".

I haven't included Tantek's suggested wording about test suites here because it's orthogonal to this issue, but I don't have any problem with it.