I don't remember offhand what the conflict is, but at least one of the doc files we install is apparently not parallel-installable. We'll need to audit to figure out what exactly is wrong.
This may be fixed as a side effect of tackling bug #226662 but I am not going to mark this yet as a duplicate because I am not 100% sure and need to check first.
This is fixed.
*** Bug 241714 has been marked as a duplicate of this bug. ***