The guide is slightly out of sync with the latest version of Sigil. The devs have been working hard to give us a better program and have recently succeeded admirably, so it may (understandably) be a bit before the guide catches up.
Anyway... a couple of things:
Sigil doesn't do anything with the page-break-(before|after) attribute. That's for the epub renderer (reader) to deal with.
The terminology dealing with "chapters" has changed a bit (and rightfully so) in the latest version of Sigil. For all intents and purpose, creating
"chapters" in Sigil means physically splitting an (x)html file into individual, smaller (x)html files. This can accomplished two ways:
1) putting the cursor just before where you want the new "chapter" to start and using 'Edit->Split at Cursor'. This takes place immediately and you now you have multiple (or more
) (x)html files. The renderer (reader) sees the new file as a hard chapter-break.
2) You can go through your document and simply mark where you want to split the file(s) using 'Insert->Split-marker'. Note that all this does is insert a horizontal rule (<hr>) with a special class-name. Marking your file this way, by itself, does nothing with regard to chapter creation. You still, at some point, need to use the 'Edit->Split at Markers' function to actually split the file(s) into "chapter" components.
Once again, I took a bit too long to post nearly
the same thing someone else did—more concisely.