[bug or doc error] partcombine and stem direction

[Graham Percival]

The docs suggest that if you \partcombine two voices (and they
have different notes), the stems will be in different directions.  The
following example (taken from the docs) have the two different
parts joined by a single stem.  Are the docs in error, or is this
a bug?

\relative c'' {
\new Staff \partcombine
  \relative g' { g g a( b) c c r r }
  \relative g' { g g r4 r e e g g }
}