Update StandalonePluginScripts.yaml (#1076)

## Changes

<!-- Please summarize your changes. -->

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.

<!-- Please link to any applicable information (forum posts, bug
reports, etc.). -->

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


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

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- [X] This contribution was created in whole or in part by me, and I
have the right to submit it under the terms of this repository's open
source licenses.
- [X] I understand and agree that this contribution and a record of it
are public, maintained indefinitely, and may be redistributed under the
terms of this repository's open source licenses.
- [X] To the best of my knowledge, all proposed changes are accurate.

---------

Author: matkeg

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

@@ -8,6 +8,11 @@ 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 Availability
+  
+  Functionality of this class may be enabled or disabled without notice. Before
+  using it, verify that it works as expected before continuing with further work.
 code_samples:
 inherits:
   - Instance