"Pages User Guide for Mac" - the section entitled "Add headers and footers in a word-processing document" is missing information

You are asking a survey style question of other users and surveys are not permitted in the Terms and conditions for these user supported communities. Just because you own a Mac does not mean you must use Pages if it is not the right tool for you. There are many alternative word processing applications from which you can choose, and most with more features than Apple implements in Pages.

The Pages User Guide for Mac represents the applicable documentation version of Pages for your operating system. For your indicated Monterey 12.6, that would be Pages v13.1.

Since there is no fundamental difference in the process of enabling headers and footers in the Documents panel for word processing or page layout documents, the writers likely chose to avoid redundancy and thus the crux of your complaint. Page layout document panel's do not incorporate top and bottom header/footer adjustment sizes. Otherwise, why duplicate the explanation? The Pages product team does not participate here, so there is no representative from that organization to explain their decision.

You can always provide direct, constructive feedback to the Pages product team from the Pages application menu.