1. Documents `ConvertVectorStore`. As the generated output is rather complex, I
have refined the comments + variable names in:
* "vector-emulate-narrow-type-unaligned-non-atomic.mlir",
to serve as reference for this pattern.
2. As a follow-on for #123527, renames `isAlignedEmulation` to `isFullyAligned`
and `numSrcElemsPerDest` to `emulatedPerContainerElem`.