Strengthen statement about "no objective system " for authorship

[ctb]

The statement "there is no objective system" reads like it could be a practical statement as in "we haven't built one yet", while I believe (and think the authors agree with me, based on the paragraph) that no such principled objective system can exist. Either way this should be clarified.

[agitter]

Three quick notes for myself as I work on this issue

• We do want to claim that no single principled objective system can exist
• The recent review about collaborative writing demonstrates different approaches, supporting that there will not be a one-size-fits-all solution (Ten simple rules for collaboratively writing a multi-authored paper #110)
• Now that we have more Manubot manuscript examples (Update example manuscripts #128), we can point to different strategies other groups have used to determine authorship and author order, including meta-review and deep-rules