using KiCad 5.1.9 on Windows 10.
Each item in the page layout also has a comment field in its properties which, are displayed in the Pl_Editor.
Entries in this field are not transferred to the * .kicad_wks file when the layout is saved.
I have empirically determined that comments are allowed in the syntax of the * .kicad_wks file.
I added comments to the items manually.
These will also be used when loading the Page Layout file and displayed in Pl_Editor.
After saving this file, however, all comments are lost again.
(setup (textsize 1.5 1.5)(linewidth 0.15)(textlinewidth 0.15)
(left_margin 10)(right_margin 5)(top_margin 5)(bottom_margin 5))
(rect (name rect1:Rect) (start 5 5 ltcorner) (end 5 5) (linewidth 0.35) (comment “Inner border”))
(rect (name rect2:Rect) (start 0 0 ltcorner) (end 0 0) (linewidth 0.7) (comment “Outer border”))