Fix documentation for setting Java I/O temp dir

This commit fixes the documentation for configuring the Java I/O temp
dir which incorrectly suggested using the -D flag as a parameter on the
command line; these flags have been removed and should now be specified
as arguments to the JVM using either the ES_JAVA_OPTS environment
variable or using the jvm.options configuration file.

Closes #20652

docs/reference/setup/sysconfig/swap.asciidoc

@@ -64,14 +64,7 @@ Systems using `systemd`::
 
 Another possible reason why `mlockall` can fail is that the temporary directory
 (usually `/tmp`) is mounted with the `noexec` option. This can be solved by
-specifying a new temp directory, by starting Elasticsearch with:
-
-[source,sh]
---------------
-./bin/elasticsearch -Djava.io.tmpdir=/path/to/temp/dir
---------------
-
-or using the `ES_JAVA_OPTS` environment variable:
+specifying a new temp directory using the `ES_JAVA_OPTS` environment variable:
 
 [source,sh]
 --------------
@@ -79,6 +72,8 @@ export ES_JAVA_OPTS="$ES_JAVA_OPTS -Djava.io.tmpdir=/path/to/temp/dir"
 ./bin/elasticsearch
 --------------
 
+or setting this JVM flag in the jvm.options configuration file.
+
 [[disable-swap-files]]
 ==== Disable all swap files