Warn about unpack's use of the temporary directory

Closes gh-15180

Author: wilkinsona

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -2948,7 +2948,12 @@ jar support which assumes that the `jruby-complete.jar` is always directly avail
 file in its own right.
 
 To deal with any problematic libraries, you can flag that specific nested jars should be
-automatically unpacked to the '`temp folder`' when the executable jar first runs.
+automatically unpacked when the executable jar first runs. Such nested jars are written
+beneath the temporary directory identified by the `java.io.tmpdir` system property.
+
+WARNING: Care should be taken to ensure that your operating system is configured so that
+it will not delete the jars that have been unpacked to the temporary directory while the
+application is still running.
 
 For example, to indicate that JRuby should be flagged for unpack using the Maven Plugin
 you would add the following configuration: