Further clarifications

Fixed non-normative text in zvfbfmin - Vector BF16 converts - to specify how narrowing conversions to BF16 could be (exactly) emulated by leveraging the round-to-odd conversion instructions.
Added some clarifications to the introduction.
Clarified the vector converts by replacing "double-width float" with "2*SEW-width format" and "single-width floar" with "SEW-width format"