Polish documentation

Add an explicit note that states that "spring.datasource.url" (or more
specifically "spring.datasource.class-name" that is inferred from the
former) is necessary to connect to a database. If the class-name isn't
specified, Spring Boot will attempt to auto-configure an embedded
database.

Closes gh-6907

Author: snicoll

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -2359,6 +2359,9 @@ DataSource configuration is controlled by external configuration properties in
 	spring.datasource.driver-class-name=com.mysql.jdbc.Driver
 ----
 
+NOTE: You should at least specify the url using the `spring.datasource.url` property or
+Spring Boot will attempt to auto-configure an embedded database.
+
 TIP: You often won't need to specify the `driver-class-name` since Spring boot can deduce
 it for most databases from the `url`.