Log Message:
Update tex-natbib{,-doc} to 8.31a.

Changes:
  v8.31a Bug fixes by Arthur Ogawa, so that the new eliding and merging features
         do not cause problems with non-REVTeX classes.
  v8.31 Bug fix: the merging features of v8.3 are supposed to be
        turned off by default but this was not completely right. With
        this bug fix, the behaviour without merging is compatible the
        earlier versions (<= 8.1).