make doc -C doc must also regenerate the PDFs

This is needed since commit 6c1dac07c4 because the PDFs are in pdf_DATA
instead dist_pdf_DATA now.

"make -C doc" builds them because all builds everything in DATA.

contrib/dist/make_dist_tarball uses "make doc -C doc"
while some other scripts use "make doc" which does "make -C doc".
Better make things uniform.