Added doc on imagecode property for agent

Author: aclement

spring-graalvm-native-docs/src/main/asciidoc/agent.adoc

@@ -42,8 +42,9 @@ specific and will need tweaking to match the package of a specific applications
 
 The access-filter.json file is specified with the `access-filter-file` option as part of the agentlib string:
 
-`-agentlib:native-image-agent=access-filter-file=access-filter.json,config-output-dir=target/classes/META-INF/native-image`
+`-Dorg.graalvm.nativeimage.imagecode=true -agentlib:native-image-agent=access-filter-file=access-filter.json,config-output-dir=target/classes/META-INF/native-image`
 
+Note the inclusion of the `-D` to set the `org.graalvm.nativeimage.imagecode` property. In normal operation a built native executable will have this property set (automatically by the image building process). If there is any code that will behave differently due to it being set (which may occur if trying to work around something not supported by GraalVM) then we should set it when running with the agent too, so that code executed whilst the agent is attached matches the code that will run in the final executable.
 
 === Using it with maven
 
@@ -154,4 +155,4 @@ generated configuration.
 to give a name to the native-image built executable. Also tweak the `<mainClass>...</mainClass>` to point to the main application class.
 
 Then, running `mvn -Pnative clean package` will do everything from end-to-end. It will compile the app,
-run the test with agent attached, finally building the native-image executable.
+run the test with agent attached, finally building the native-image executable.