rust-lang/portable-simd#272: Standardize documentation for SIMD vector and mask types

This is a documentation-only patch that standardizes the presentation of vector types like `u32x4` and mask types like `mask32x16`.

The reasoning behind the patch was as follows:
1. Standardized terminology should be preferred, so `element` instead of `value` and `SIMD vector` instead of `vector`. These terms appear in the rest of the documentation and tutorials.
2. Try to avoid situations where two numbers are next to each other. So `16 elements of 32 bits` instead of `16 32-bit elements`.
4. Try to anticipate what readers are looking for -- so state the full bit-width directly.

### Vector Types
- Before: Vector of 32 `i8` values
- After: A 256-bit SIMD vector with 32 elements of type `i8`.

### Mask Types
- Before: Vector of 16 16-bit masks
- After: A mask for SIMD vectors with 16 elements of 32 bits.

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