Add clarification for hibernate docs (#1566)
Add clarification for hibernate docs (#1566)

Adds some clarification for setting the database connection URL in the Spanner + Hibernate sample documentation.
GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples,GoogleCloudPlatform/java-docs-samples