the Coq software is DFSG-free provided by the src:coq package. But its
documentation is under the OPL-1.0 -- hence not DFSG-free because of
restrictions on modification [see 1 below].
In the users' interest this documentation has hence been split in a
non-free src:coq-doc package providing binary packages coq-doc-html and
coq-doc-pdf -- with a third one coq-doc to get both.
That means there's no problem with building such a package on the
buildd network. I propose to add "XS-Autobuild: yes" to its
debian/control.