Merge pull request #4917 from FlowFuse/default-stack-docs

Add note to Instance Types setting default Stack

docs/admin/introduction.md

@@ -129,6 +129,8 @@ Whenever a new Instance Type is created, it will need to be manually enabled for
 the individual [Team Types](#managing-team-types) before they will be available
 for teams to use.
 
+Instance Types also control the default Stack (and in turn, the default Node-RED versions) that Instances will run with if there are multiple available Stacks associated with an Instance Type.
+
 ### Managing Stacks
 
 > Admin Settings > Stacks