add jvm clarification (elastic#57460)

Emphasise in the Docker documentation that although the default heap size is
1GB, the docker-compose.yml example specifies 512MB.

docs/reference/setup/install/docker.asciidoc

@@ -283,7 +283,7 @@ method, you can also configure this by using the `ES_JAVA_OPTS` environment
 variable to set the heap size. For example, to use 16 GB, specify
 `-e ES_JAVA_OPTS="-Xms16g -Xmx16g"` with `docker run`. Note that while the
 default root `jvm.options` file sets a default heap of 1 GB, any value you set
-in `ES_JAVA_OPTS` will override it.
+in `ES_JAVA_OPTS` will override it. The `docker-compose.yml` file above sets the heap size to 512 MB.
 
 IMPORTANT: You must <<heap-size,configure the heap size>> even if you are
 https://docs.docker.com/config/containers/resource_constraints/#limit-a-containers-access-to-memory[limiting