doc: speed up dot generation

dot supports generating multiple graphs in one run, that was not always
the case and Doxygen had a default of not using that feature.  Given it
got introduced in dot 1.8.10 and Wikimedia CI server has 2.26.x, I guess
it is safe to enable this.

End result, is less shell out overhead.

Change-Id: Ia07b430bd1af608bbe9c10eecbe2df3e0b9c74fc