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.'

Fixes cplusplus#7430
Fixes cplusplus/papers#670

Author: mattkretz