Some minor docs improvements, as I dug through this codebase recently:

• I wanted to clarify, "Does this CTE only provision Crucible datasets to U.2s, and not M.2s?". The answer to this question is: "yes, but implicitly decided when we provision the datasets themselves". Added a comment to clarify this during region selection.
• Additionally, when referencing a virtual table from an earlier WITH candidate_zpools AS ... arm of the CTE, make that reference explicit. This is a no-op change, but it clarifies the dependencies within the CTE.