[DE-192] consider overwriteMode only in case of SaveMode.Append (#950)
[DE-192] consider overwriteMode only in case of SaveMode.Append (#950)

* updated overwriteMode defaults and applicability

* Update spark-connector-new.md

Co-authored-by: ansoboleva <88632894a9e531c470067a10e46d4700ecadd961@users.noreply.github.com>
arangodb/docs,arangodb/docs,arangodb/docs,arangodb/docs