[flang] Document extension: Ew.0 output editing

The restrictions on the relationship between a "kP" scale factor
and a Ew.d/Dw.d/Gw.d significant digit count are not enforced for
output editing when there is no nonzero scale factor in effect.
(An error message about a bad scale factor is confusing if no kP
control edit descriptor has been used.)  Document this usage in
Extensions.md.

Differential Revision: https://reviews.llvm.org/D127435