Document @LiquibaseDataSource feature

Closes gh-8214

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/liquibase/LiquibaseAutoConfiguration.java

 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -76,11 +76,12 @@ public class LiquibaseAutoConfiguration {
 		private final DataSource liquibaseDataSource;
 
 		public LiquibaseConfiguration(LiquibaseProperties properties,
-				ResourceLoader resourceLoader, DataSource dataSource,
+				ResourceLoader resourceLoader,
+				ObjectProvider<DataSource> dataSourceProvider,
 				@LiquibaseDataSource ObjectProvider<DataSource> liquibaseDataSourceProvider) {
 			this.properties = properties;
 			this.resourceLoader = resourceLoader;
-			this.dataSource = dataSource;
+			this.dataSource = dataSourceProvider.getIfUnique();
 			this.liquibaseDataSource = liquibaseDataSourceProvider.getIfAvailable();
 		}
 

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

@@ -2210,6 +2210,12 @@ The master change log is by default read from `db/changelog/db.changelog-master.
 can be set using `liquibase.change-log`. In addition to YAML, Liquibase also supports
 JSON, XML, and SQL change log formats.
 
+By default Liquibase will autowire the (`@Primary`) `DataSource` in your context and use
+that for migrations. If you like to use a different `DataSource` you can create one and
+mark its `@Bean` as `@LiquibaseDataSource` - if you do that remember to create another one
+and mark it as `@Primary` if you want two data sources. Or you can use Liquibase's native
+`DataSource` by setting `liquibase.[url,user,password]` in external properties.
+
 See
 {sc-spring-boot-autoconfigure}/liquibase/LiquibaseProperties.{sc-ext}[`LiquibaseProperties`]
 for details of available settings like contexts, default schema etc.