Document how to use spring.factories to add auto-configuration to a test slice

[wilkinsona]

#6001 and #6335 made it possible to add auto-configuration to a test slice by listing it in spring.factories but it was only documented in the javadoc of @ImportAutoConfiguration. Prompted by a discussion on Gitter with @jnizet, we should also mention it in the reference documentation. The "Additional Auto-configuration and Slicing" section is probably the right place for it.