.Net: Update the description to explain why context.Variables.Update …

…is being used (#3078)

### Motivation and Context

Provide some additional information in the notebook to help users
understand when to use `context.Variables.Update`

Resolves #729

### Description

### Contribution Checklist

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

Co-authored-by: Dmytro Struk <13853051+dmytrostruk@users.noreply.github.com>

dotnet/notebooks/04-context-variables-chat.ipynb

@@ -191,7 +191,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Update the history with the output"
+    "Update the history with the output and set this as the new input value for the next request"
    ]
   },
   {