Add comment explaining priority of JVM options (#52348)

Reading the startup scripts does not elucidate how JVM options are
applied. Instead, the reader must consult the source for the JVM options
parser. This commit adds some transparency around this process so that
it easier to understand reading the startup scripts how the final JVM
options to start Elasticsearch are constructed.

distribution/src/bin/elasticsearch

@@ -8,8 +8,7 @@
 #   ES_PATH_CONF -- Path to config directory
 #   ES_JAVA_OPTS -- External Java Opts on top of the defaults set
 #
-# Optionally, exact memory values can be set using the `ES_JAVA_OPTS`. Note that
-# the Xms and Xmx lines in the JVM options file must be commented out. Example
+# Optionally, exact memory values can be set using the `ES_JAVA_OPTS`. Example
 # values are "512m", and "10g".
 #
 #   ES_JAVA_OPTS="-Xms8g -Xmx8g" ./bin/elasticsearch
@@ -46,6 +45,13 @@ then
   fi
 fi
 
+# The JVM options parser produces the final JVM options to start Elasticsearch.
+# It does this by incorporating JVM options in the following way:
+#   - first, system JVM options are applied (these are hardcoded options in the
+#     parser)
+#   - second, JVM options are read from jvm.options and jvm.options.d/*.options
+#   - third, JVM options from ES_JAVA_OPTS are applied
+#   - fourth, ergonomic JVM options are applied
 ES_JAVA_OPTS=`export ES_TMPDIR; "$JAVA" -cp "$ES_CLASSPATH" org.elasticsearch.tools.launchers.JvmOptionsParser "$ES_PATH_CONF"`
 
 # manual parsing to find out, if process should be detached