[LyX/master] Add documentation for the new layout attributes.

[Pavel Sanda]

On Wed, Jul 08, 2020 at 06:54:20PM +0200, Thibaut Cuvelier wrote:
> > > Here is the updated version of the manual and the corresponding patch. It
> > > should apply cleanly, now (even though I don't get why it's that
> > large???)!
> >
> > I don't want updated version. I want some very old one where you did not
> > made any
> > git merging or rebasing.
> >
> 
>  Here is a very old copy, I've updated it so that it reflects the current
> state of the code. Is it good enough?

Good, do you have idea on top of which commit this was written (even approximate
date would be helpful).

Pavel