S57 doc: LNAM_REFS is ON by default

git-svn-id: https://svn.osgeo.org/gdal/trunk@27292 f0d54148-0727-0410-94bb-9a71ac55c965
S57: make it possible to set LNAM_REFS=OFF as advertized in the doc

git-svn-id: https://svn.osgeo.org/gdal/trunk@27293 f0d54148-0727-0410-94bb-9a71ac55c965