For SwDoc::AppendDoc we use the StartOfContent node as the
starting copy node to prevent merging of the first node in
CopyRange and to get a 2nd node needed for CopyRange in case
of single content node documents.

This correctly counts StartOfContent as a non-copy node when
adapting the bookmark ranges for copying.

Change-Id: Ia3ee0328a1be5548f8751aa2240812c4662fb73f
Reviewed-on: https://gerrit.libreoffice.org/20383Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>