[SQL] Document/fix text table (TextIO) insert behavior

Tables of type `text` are backed by TextIO. We don't do any extra configuration, so it results in behavior like this:

 - create text table with location 'test-file.csv'

 - if the file exists, you can select from it;

 - now insert values into that table;

     - this results in TextIO writing files with names like 'test-file.csv-00000-of-00001';

     - selecting from the original table doesn't allow selecting from the newly created files;

We need to properly document this, possibly log to Stdout.

Imported from Jira [BEAM-4815](https://issues.apache.org/jira/browse/BEAM-4815). Original Jira may contain additional context.
Reported by: kedin.