Reinsert paragraph about heap size

This paragraph was accidentally dropped when preparing to merge
setting the heap size.

docs/reference/setup/jvm-options.asciidoc

@@ -1,6 +1,10 @@
 [[jvm-options]]
 === Setting JVM options
 
+You should rarely need to change Java Virtual Machine (JVM) options. If you do,
+the most likely change is setting the <<heap-size,heap size>>. The remainder of
+this document explains in detail how to set JVM options.
+
 The preferred method of setting JVM options (including system properties and JVM
 flags) is via the `jvm.options` configuration file. The default location of this
 file is `config/jvm.options` (when installing from the tar or zip distributions)