Update StandalonePluginScripts.yaml

Changes are based on the following comments and personal observations of the instance's functionality: 

https://devforum.roblox.com/t/insert-object-menu-does-not-have-standalonepluginscripts/1161930/4?u=matkeftw
https://devforum.roblox.com/t/selection-doesnt-work-with-standalonepluginscripts/1204790/2?u=matkeftw

Additional information has been added to document the instance's inconsistent behavior. This info should be useful, as Studio does not provide any additional info on this, nor is there any additional information online.

Author: matkeg

content/en-us/reference/engine/classes/StandalonePluginScripts.yaml

@@ -8,6 +8,15 @@ description: |
   in a standalone plugin `Class.DataModel` when Roblox Studio starts (it is not
   useful outside the context of plugins). When a place to be edited is loaded in
   Roblox Studio, scripts inside this container are not run.
+
+  #### Limited Functionality and Availabilty
+  
+  Throughout the history of this instance, its functionality has been enabled
+  and disabled without notice. Before doing any work with it, make sure to verify
+  that it works as expected before continuing with further work.
+
+  Seemingly, when enabled, **StandalonePluginScripts** work only with Local 
+  `Class.Plugin|Plugins`.
 code_samples:
 inherits:
   - Instance