[memref][Transform][NFC] Improve the doc for masked_vectorize

The `transform.structured.masked_vectorize` operator assumes that the
iteration space of the given linalg op is smaller than the given vector
sizes.
Explicitly states this requirement in the description of the operation.

Also fix the related comment and assert message in the vectorization code.
The wording was flipped.

NFC

Differential Revision: https://reviews.llvm.org/D142628