Document using xpack.ml.use_auto_machine_memory_percent setting in docker getting started (elastic#114009)

If xpack.ml.use_auto_machine_memory_percent is not explicitly set to true then
the default value (false) means ML will only use 30% of the available memory making
it impractical to run the ELSER model. This is useful for users wanting to get started 
with semantic search.The single node docker instructions have been updated with a 
command that gives the container enough memory to run the ELSER model and enables xpack.ml.use_auto_machine_memory_percent. For the multi-node guide the docker 
compose file is updated to enable the ml setting for every node in the cluster.

Author: davidkyle

docs/reference/setup/install/docker.asciidoc

@@ -86,6 +86,15 @@ docker run --name es01 --net elastic -p 9200:9200 -it -m 1GB {docker-image}
 TIP: Use the `-m` flag to set a memory limit for the container. This removes the
 need to <<docker-set-heap-size,manually set the JVM size>>.
 +
+
+{ml-cap} features such as <<semantic-search-elser, semantic search with ELSER>>
+require a larger container with more than 1GB of memory.
+If you intend to use the {ml} capabilities, then start the container with this command:
++
+[source,sh,subs="attributes"]
+----
+docker run --name es01 --net elastic -p 9200:9200 -it -m 6GB -e "xpack.ml.use_auto_machine_memory_percent=true" {docker-image}
+----
 The command prints the `elastic` user password and an enrollment token for {kib}.
 
 . Copy the generated `elastic` password and enrollment token. These credentials

docs/reference/setup/install/docker/docker-compose.yml

@@ -90,6 +90,7 @@ services:
       - xpack.security.transport.ssl.certificate_authorities=certs/ca/ca.crt
       - xpack.security.transport.ssl.verification_mode=certificate
       - xpack.license.self_generated.type=${LICENSE}
+      - xpack.ml.use_auto_machine_memory_percent=true
     mem_limit: ${MEM_LIMIT}
     ulimits:
       memlock:
@@ -130,6 +131,7 @@ services:
       - xpack.security.transport.ssl.certificate_authorities=certs/ca/ca.crt
       - xpack.security.transport.ssl.verification_mode=certificate
       - xpack.license.self_generated.type=${LICENSE}
+      - xpack.ml.use_auto_machine_memory_percent=true
     mem_limit: ${MEM_LIMIT}
     ulimits:
       memlock:
@@ -170,6 +172,7 @@ services:
       - xpack.security.transport.ssl.certificate_authorities=certs/ca/ca.crt
       - xpack.security.transport.ssl.verification_mode=certificate
       - xpack.license.self_generated.type=${LICENSE}
+      - xpack.ml.use_auto_machine_memory_percent=true
     mem_limit: ${MEM_LIMIT}
     ulimits:
       memlock: