Clarify messages. Book Creator PDF rendering died

Per https://www.mediawiki.org/wiki/Reading/Web/PDF_Functionality#Update_on_PDF_rendering,_July_15_2019

"In terms of books, we've left it in the hands of volunteer developers and PediaPress.
We'll be glad to reach out to them with questions, but we're not planning any involvement
in terms of the technical implementation."

Change-Id: I170483d18947464ef6f82c9eb6ea9d6666b8c713