[expr.prim.req] Applying wording fixes.

Join paragraphs to eliminate unnecessary single-liners.
Various formatting fixes.
[expr] Update the list of unvevaluated operands in p8.
[expr.prim.req] Add a note to ad hoc constraints.

Also fix a broken reference.
[expr.prim.req] Applying wording fixes.
Merge branch 'master' of https://github.com/cplusplus/concepts-ts