Yes: Put it at the top of the page Yes: Not having a summary if the first paragraph works fine\. Yes: "Paragraph long explanation summarizing the contents of the page\.\.\." Yes: "Using Markdown as normal\." Yes: "If the page has a vote, describe exactly what the vote is about\."
Note that this is an admin-only feature for now. So maybe should be left off the style guide or marked in some way so that people aren't confused about it.