remove page_down and page_up from docs
remove page_down and page_up from docs

I'm not sure if it was intentional, but the `page_down` and `page_up` edit commands have not worked for a while.  Now, serde auto-converts the variants to snake_case strings, and there is only `scroll_page_up` and `scroll_page_down`.
google/xi-editor,google/xi-editor,google/xi-editor,google/xi-editor