Auto merge of #41859 - froydnj:align-float-parts, r=sfackler

fix confusion about parts required for float formatting

The documentation for flt2dec doesn't match up with the actual
implementation, so fix the documentation to align with reality.
Presumably due to the mismatch, the formatting code for floats in
std::fmt can use correspondingly shorter arrays in some places, so fix
those places up as well.

Fixes #41304.