[SPARK-19948] Document that saveAsTable uses catalog as source of truth for table existence.

It is quirky behaviour that saveAsTable to e.g. a JDBC source with SaveMode other
than Overwrite will nevertheless overwrite the table in the external source,
if that table was not a catalog table.

Author: Juliusz Sompolski <julek@databricks.com>

Closes #17289 from juliuszsompolski/saveAsTableDoc.