Clarify use of ES_JAVA_OPTS and Docker (#51987)

Backport of #51867.

Tweak the documentation around configuring the heap size when using
Docker, to state that:

- using `ES_JAVA_OPTS` is the preferred method
- Any `ES_JAVA_OPTS` overrides the defaults in `jvm.options`
- It's possible to bind-mount a custom `jvm.options`

docs/reference/setup/install/docker.asciidoc

@@ -299,6 +299,7 @@ docker run <various parameters> bin/elasticsearch -Ecluster.name=mynewclusternam
 
 See <<configuring-tls-docker>>.
 
+[[docker-notes-for-production]]
 ==== Notes for production use and defaults
 
 We have collected a number of best practices for production use.
@@ -367,13 +368,20 @@ published ports with `--publish-all`, unless you are pinning one container per
 host.
 +
 . Use the `ES_JAVA_OPTS` environment variable to set heap size. For example, to
-use 16GB use `-e ES_JAVA_OPTS="-Xms16g -Xmx16g"` with `docker run`.
+use 16GB use `-e ES_JAVA_OPTS="-Xms16g -Xmx16g"` with `docker run`. Note
+that while the default configuration file `jvm.options` sets a default heap
+of 1GB, any value you set in `ES_JAVA_OPTS` will override it.
 +
 --
 NOTE: You still need to <<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
 memory access] to the container.
 --
+While setting the heap size via an environment variable is the recommended
+method, you can also configure this by bind-mounting your own `jvm.options`
+file under `/usr/share/elasticsearch/config/`. The file that {es} provides
+contains some important settings, so you should start by taking a copy of
+`jvm.options` from an {es} container and editing it as you require.
 
 . Pin your deployments to a specific version of the {es} Docker image. For
 example, +docker.elastic.co/elasticsearch/elasticsearch:{version}+.

docs/reference/setup/jvm-options.asciidoc

@@ -11,6 +11,10 @@ file is `config/jvm.options` (when installing from the tar or zip distributions)
 and `/etc/elasticsearch/jvm.options` (when installing from the Debian or RPM
 packages).
 
+NOTE: If you are using the <<docker,Docker distribution of {es}>>, we
+recommend that you <<docker-notes-for-production,set the heap size using the
+`ES_JAVA_OPTS` environment variable>>.
+
 This file contains a line-delimited list of JVM arguments following
 a special syntax: