Although did before a change of the page layout within one Writer document from vertical to horizontal, and back to vertical, I could not find the information in the help file. The help file needs to get the information: Change from vertical page layout to horizontal page layout within one document: Create a horizontal page layout in Styles and Formatting > Page Next insert manual break with format change Insert > Manual break > page break and select Style: select the page style name created before for the horizontal page format. (The style used above might no be matching the Help file style.) Should this explanation be in the Help file, then the search with keywords needs to be improved. eg. "horizontal page layout", "horizontal page", "change vertical to horizontal", "vertical to horizontal", etc should lead to the above sketch text.
The change of page orientation is described at this help page: https://help.libreoffice.org/Writer/Changing_Page_Orientation_Landscape_or_Portrait You can get there by index entries "landscape and portrait" or "pages > orientation". Would you like to add anything to the help page?
I don't know if that help file needs some help or not, but it definitely exists. Therefore I close this bug as invalid.
The link @Stanislav Horacek posted is ok. I was meant the off-line Help file. Please have look and see it is worth to re-open the bug.