Polish "Use custom DataSource if Flyway or Liquibase has user or url"

Closes gh-11751

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

@@ -2139,7 +2139,10 @@ uses that for migrations. If you like to use a different `DataSource`, you can c
 one and mark its `@Bean` as `@FlywayDataSource`. If you do so and want two data sources,
 remember to create another one and mark it as `@Primary`. Alternatively, you can use
 Flyway's native `DataSource` by setting `spring.flyway.[url,user,password]`
-in external properties.
+in external properties. Setting either `spring.flyway.url` or `spring.flyway.user`
+is sufficent to cause Flyway to use its own `DataSource`. If any of the three
+properties has not be set, the value of its equivalent `spring.datasource` property will
+be used.
 
 There is a {github-code}/spring-boot-samples/spring-boot-sample-flyway[Flyway sample] so
 that you can see how to set things up.
@@ -2175,7 +2178,10 @@ that for migrations. If you need to use a different `DataSource`, you can create
 mark its `@Bean` as `@LiquibaseDataSource`. If you do so and you want two data sources,
 remember to create another one and mark it as `@Primary`. Alternatively, you can use
 Liquibase's native `DataSource` by setting `spring.liquibase.[url,user,password]` in
-external properties.
+external properties. Setting either `spring.liquibase.url` or `spring.liquibase.user`
+is sufficent to cause Liquibase to use its own `DataSource`. If any of the three
+properties has not be set, the value of its equivalent `spring.datasource` property will
+be used.
 
 See
 {sc-spring-boot-autoconfigure}/liquibase/LiquibaseProperties.{sc-ext}[`LiquibaseProperties`]