DOC import: fix nested comments

See 2.9.70 (FBKF) in the spec, the ibkl field should be used to find the
position in PlcfBkf.

Previously we assumed that PlcfBkl and PlcfBkf is parallel for
annotations, as but that's not true for nested comments.

Change-Id: I76a6a757b41d2f9b7fe7fe9c9e01a92c251f7b32
(cherry picked from commit 5cfda4f4de8da82485e27b1f0f0d94211d3b4d95)
Reviewed-on: https://gerrit.libreoffice.org/10138
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>