Document buildpacks are configured with target Java version

Closes gh-21796

Author: snicoll

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/asciidoc/packaging-oci-image.adoc

@@ -77,6 +77,10 @@ The following table summarizes the available properties and their default values
 | `false`
 |===
 
+NOTE: The plugin detects the target Java compatibility of the project using the JavaPlugin's `targetCompatibility` property.
+By default, the plugin instructs the buildpacks to install the same Java version.
+You can override this behaviour by setting the `BP_JVM_VERSION` environment variable, see <<build-image-example-builder-configuration,builder configuration>>.
+
 
 
 [[build-image-examples]]

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/docs/asciidoc/packaging-oci-image.adoc

@@ -101,7 +101,11 @@ The following table summarizes the available parameters and their default values
 | `false`
 |===
 
-For more details, see <<build-image-example-custom-image-builder,custom image builder>> and <<build-image-example-custom-image-name,custom image name>>.
+NOTE: The plugin detects the target Java compatibility of the project using the compiler's plugin configuration or the `maven.compiler.target` property.
+By default, the plugin instructs the buildpacks to install the same Java version.
+You can override this behaviour by setting the `BP_JVM_VERSION` environment variable, see <<build-image-example-builder-configuration,builder configuration>>.
+
+For more details, see also <<build-image-example-custom-image-builder,custom image builder>> and <<build-image-example-custom-image-name,custom image name>>.
 
 include::goals/build-image.adoc[leveloffset=+1]