Clarify usage of jvm.options in its comments

Since elastic#51882 we recommend not editing the `jvm.options` file, preferring
instead to override its contents with additional files in
`jvm.options.d`. However the inline comments in this file do not point
users in that direction. This commit adjusts these inline comments.

distribution/src/config/jvm.options

@@ -1,12 +1,29 @@
-## JVM configuration
+################################################################
+##
+## JVM Options
+##
+################################################################
+##
+## WARNING: DO NOT EDIT THIS FILE. If you want to override the
+## JVM options in this file, or set any additional options, you
+## should create one or more files in the jvm.options.d
+## directory containing your adjustments.
+##
+## See https://www.elastic.co/guide/en/elasticsearch/reference/current/jvm-options.html
+## for more information.
+##
+################################################################
+
+
 
 ################################################################
 ## IMPORTANT: JVM heap size
 ################################################################
 ##
-## You should always set the min and max JVM heap
-## size to the same value. For example, to set
-## the heap to 4 GB, set:
+## You must always set the initial and maximum JVM heap size to
+## the same value. For example, to set the heap to 4 GB, create
+## a new file in the jvm.options.d directory containing these
+## lines:
 ##
 ## -Xms4g
 ## -Xmx4g
@@ -16,19 +33,23 @@
 ##
 ################################################################
 
-# Xms represents the initial size of total heap space
-# Xmx represents the maximum size of total heap space
+# Xms represents the initial size of the JVM heap
+# Xmx represents the maximum size of the JVM heap
 
 -Xms${heap.min}
 -Xmx${heap.max}
 
+
+
 ################################################################
 ## Expert settings
 ################################################################
 ##
-## All settings below this section are considered
-## expert settings. Don't tamper with them unless
-## you understand what you are doing
+## All settings below here are considered expert settings.  Do
+## not tamper with them unless you understand what you are
+## doing. If you want to adjust any of these settings then do
+## not do so by editing this file; instead, create a new file in
+## the jvm.options.d directory to override these settings.
 ##
 ################################################################
 
@@ -51,8 +72,9 @@
 
 ## heap dumps
 
-# generate a heap dump when an allocation from the Java heap fails
-# heap dumps are created in the working directory of the JVM
+# generate a heap dump when an allocation from the Java heap fails; heap dumps
+# are created in the working directory of the JVM unless an alternative path is
+# specified
 -XX:+HeapDumpOnOutOfMemoryError
 
 # specify an alternative path for heap dumps; ensure the directory exists and