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GH-37889: [Java][Doc] Improve JDBC driver documentation (#38469)

### Rationale for this change
Correct inaccuracies and better document how to connect with the Flight SQL JDBC Driver.

### What changes are included in this PR?
- Correct the user property.
- Correct the default for useEncryption.
- Elaborate on using the Properties object when connecting and describe the order of precedence.

### Are these changes tested?
N/A

### Are there any user-facing changes?
Yes
* Closes: #37889 

Authored-by: James Duong <james.duong@improving.com>
Signed-off-by: David Li <li.davidm96@gmail.com>