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

[Pavel Sanda]

On Wed, Jul 08, 2020 at 09:17:12AM +0200, Pavel Sanda wrote:
> commit 7989ebca85ddf95d58dfb86e234a17e5470d62c2
> Author: Thibaut Cuvelier <cuvelier.thibaut at gmail.com>
> Date:   Fri May 10 02:54:44 2019 +0200
> 
>     Add documentation for the new layout attributes.

Thibaut, this patch broke the manual (likely wrong merge from some changes
already committed to master.) I already reverted it.

Can you send either pacth which works or old version of your manual
which was fully working, I can do careful merge.

Pavel