Add experimental wording for positron notebooks setting (#5025)

The wording on the notebook setting was very nonchalant before. Now we
match other positron experimental features by making it as clear as
possible that the notebooks are not ready for primetime yet.

<img width="906" alt="image"
src="https://github.com/user-attachments/assets/d3fb0c8f-ab31-42d5-94de-56fc5ac923a1">


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### QA Notes
The setting key was changed to
`positron.notebooks.usePositronNotebooksExperimental` from
`positron.notebooks.usePositronNotebooks`. Which could cause issues it
the tests manually set this flag. There doesn't appear to be any
instances of this in the main positron repo, though.

src/vs/workbench/contrib/positronNotebook/browser/positronNotebook.contribution.ts

@@ -35,7 +35,7 @@ import { IPositronNotebookInstance } from 'vs/workbench/services/positronNoteboo
 /**
  * Key for the configuration setting that determines whether to use the Positron Notebook editor
  */
-const USE_POSITRON_NOTEBOOK_EDITOR_CONFIG_KEY = 'positron.notebooks.usePositronNotebooks';
+const USE_POSITRON_NOTEBOOK_EDITOR_CONFIG_KEY = 'positron.notebooks.usePositronNotebooksExperimental';
 
 /**
  * Retrieve the value of the configuration setting that determines whether to use the Positron
@@ -55,7 +55,10 @@ Registry.as<IConfigurationRegistry>(ConfigurationExtensions.Configuration).regis
 		[USE_POSITRON_NOTEBOOK_EDITOR_CONFIG_KEY]: {
 			type: 'boolean',
 			default: false,
-			markdownDescription: localize('positron.usePositronNotebooks', "Should the Positron Notebook editor be used instead of the default one?"),
+			markdownDescription: localize(
+				'positron.usePositronNotebooks',
+				"Use experimental Positron Notebook editor.\n\n**CAUTION**: The Positron Notebook editor is experimental and does not yet support all notebook features."
+			),
 		}
 	}
 });