I'm writing some documentation for a project of mine. This project depends on a GNU project so I must license it under the GNU GPL. In future, it might be integrated in that GNU project so I'm licensing the manual under the FDL. I've copied some parts of the manual of the GNU project into my manual. I've already included the Invariant Sections (the GNU GPL and FDL chapters, which I'd need without any parts of the other manual too). What do I have to do? Should I say that I grabbed some parts of that manual and cite the authors in the colophon? Is it sufficient?
And regarding Front/Back-Cover texts? Do I have to put them in some specific places?
EDIT: How much text has to be copied to make my manual a "Modified Version" of the document? It isn't stated in the license.
Sorry for my bad English.