Document usage of auto-configuration AP for 3rd party starters

Closes gh-13368

Author: snicoll

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

@@ -6490,6 +6490,30 @@ TIP: You should mark the dependencies to the library as optional so that you can
 the autoconfigure module in your projects more easily. If you do it that way, the library
 won't be provided and Spring Boot will back off by default.
 
+Spring Boot uses an annotation processor to collect the conditions on auto-configurations
+in a metadata file (`META-INF/spring-autoconfigure-metadata.json`). If that file is
+present, it is used to eagerly filter auto-configurations that do not match, which will
+improve startup time. It is recommended to add the following dependency in a module that
+contains auto-configurations:
+
+[source,xml,indent=0,subs="verbatim,quotes,attributes"]
+----
+	<dependency>
+		<groupId>org.springframework.boot</groupId>
+		<artifactId>spring-boot-autoconfigure-processor</artifactId>
+		<optional>true</optional>
+	</dependency>
+----
+
+With Gradle you would add:
+
+[source,groovy,indent=0,subs="verbatim,quotes,attributes"]
+----
+	dependencies {
+		compileOnly "org.springframework.boot:spring-boot-autoconfigure-processor"
+	}
+----
+
 
 
 [[boot-features-custom-starter-module-starter]]