Document FlywayMigrationStrategy

Closes gh-5656

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

@@ -1851,7 +1851,10 @@ underscore-separated version, e.g. '`1`' or '`2_1`'). By default they live in a
 the Flyway class from flyway-core for details of available settings like schemas etc. In
 addition Spring Boot provides a small set of properties in
 {sc-spring-boot-autoconfigure}/flyway/FlywayProperties.{sc-ext}[`FlywayProperties`]
-that can be used to disable the migrations, or switch off the location checking.
+that can be used to disable the migrations, or switch off the location checking. Spring
+Boot will call `Flyway.migrate()` to perform the database migration. If you would like
+more control, provide a `@Bean` that implements
+{sc-spring-boot-autoconfigure}/flyway/FlywayMigrationStrategy.{sc-ext}[`FlywayMigrationStrategy`].
 
 TIP: If you want to make use of http://flywaydb.org/documentation/callbacks.html[Flyway
 callbacks], those scripts should also live in the `classpath:db/migration` folder.