P1928R15 std::simd --- merge data-parallel types from the Parallelism TS 2

* Add closing parenthesis in [simd.reductions] p3.
* Add namespace std to simd_flags, basic_simd, and basic_simd_mask
* Add comma after 'e.g.'
* Add \rSec3[simd.expos.defn]{Exposition-only helpers} (missing sibling)
* Remove \iref from simd.expos.abi] (5.3), already in (5.2)

Fixes cplusplus#7430
Fixes cplusplus/papers#670

Author: mattkretz