doc: update FAQ to mention aqa-tests branches (adoptium#3564)

* doc: update FAQ to mention aqa-tests branches

Signed-off-by: Stewart X Addison <sxa@redhat.com>

* Update following review

Signed-off-by: Stewart X Addison <sxa@redhat.com>

---------

Signed-off-by: Stewart X Addison <sxa@redhat.com>

FAQ.md

@@ -201,7 +201,11 @@ an individual test for example:
 `test/jdk/java/lang/invoke/lambda/LambdaFileEncodingSerialization.java`
 
 If you then need to run manually on the machine itself (outside jenkins)
-then the process is typically like this:
+then the process is typically like this.  To avoid the test material not
+matching the JDK under test which can lead to false failures when you're
+testing a build which isn't the latest (such as a previous GA/the last
+release), it is recommended that you check out the appropriate branch for
+the last release from the aqa-tests repository in the first line here.
 
 ```sh
 git clone https://github.com/adoptium/aqa-tests && cd aqa-tests