I was asked to review the LINEST description with a special view of the correct use of the subscripts. I have attached the changes as an odf file containing the LINEST description with the required changes. I have enabled track changes so that the changes are easier viewable.
I have made one, likely controversial, change:
knownX is optional and we currently just say that by default it is the sequence 1 through k. Since knownX has to be a correctly sized array for the rest of the description to make sense, there are two natural possibilities: the entries could be given in column-major or row-major order. I have specified them as row-major. That is what Excel 2016 and Gnumeric 1.12.40 are doing. Libreoffice 188.8.131.52 appears to use column major order instead.
Alternatively we could specify "column-major or row-major order with the choice being implementation defined".