Add rounding mode support in charconv to design decisions

[mborland]

Matt Borland wrote:

I think the best move here is to add deviations into the design decisions page which becomes "Design Decisions and Standards Deviations". Right now there are some, but not all, that are sprinkled throughout the documentation. The could all be consolidated in one place, since the rationales for deviations are design decisions.

While C's strtod, strtod32, etc. respect the rounding mode (at least
in environments that define STDC_IEC_60559*_), std::from_chars
does not.
It is a reasonable and practical deviation from the standard that
Boost.Decimal's from_chars does respect the rounding mode.

Regards,
Michel