Improve handling of multi-modal node labels

[skieffer]

Nodes can define both latex and docref labels. The behavior we want is as follows:

• if both docref and latex are provided, we want the latter to
  make the label, and the former only to navigate a linked doc

• if only a docref is provided, we want it to provide both navigation,
  and the label

However, at present the "both" case is incorrect, in that the label is showing
a combination of the latex and docref rendered labels.

RELEASE NOTES

Improvements

• When nodes define both latex and docref labels, the latter now only serves
  to navigate a linked doc; it contributes to the label only when no latex
  label is defined.