[MINOR] Improve SQLConf messages

### What changes were proposed in this pull request?
The current SQLConf messages of `spark.sql.hive.convertMetastoreParquet` and `spark.sql.hive.convertMetastoreOrc` are not very clear to end users. This PR is to improve them.

### How was this patch tested?
N/A

Author: gatorsmile <gatorsmile@gmail.com>

Closes #18657 from gatorsmile/msgUpdates.