Clarify Simulated Group (#1200)

<!-- Ticket number or summary of work -->
## Summary
Adds a reference to more clearly define what the Simulated Group is and
means to editing this component in Makeswift.

I know this makes things slightly redundant because you are told to
select a Simulated Group before adding content for Group A just below
this. However, both myself and a merchant's developer missed this the
first time through and were very confused as to why we had Targeted
Groups but content didn't appear for them, so I think a further
definition alongside Simulated Group's definition is warranted. It's not
really clear that this is a requirement to create the desired content,
in my opinion, without that.


## What changed?
<!-- Provide a bulleted list in the present tense -->
* A note is added below the Simulated Group definition that clearly
defines why it must be selected.

## Release notes draft
<!-- Provide an entry for the release notes using simple, conversational
language. Don't be too technical. Explain how the change will benefit
the merchant and link to the feature.

Examples:
* The newly-released [X feature] is now available to use. Now, you’ll be
able to [perform Y action].
* We're happy to announce [X feature], which can help you [perform Y
action].
* [X feature] helps you to create [Y response] using the [Z query
parameter]. Now, you can deliver [ex, localized shopping experiences for
your customers].
* Fixed a bug in the [X endpoint]. Now the [Y field] will appear when
you click [Z option]. -->
* 
N/A
## Anything else?
<!-- Add related PRs, salient notes, additional ticket numbers, etc. -->
N/A

Author: bc-RyanGross

docs/storefront/catalyst/getting-started/guides/customer-group-component.mdx

@@ -55,7 +55,9 @@ The goal here is to add a dynamic section on the page with content that changes
 The controls for this component on the right handle the following:
 
 - **Selected groups**: Choose from a list of available customer groups that you’d like to create custom content for.
-- **Simulated group:** This will control which available groups you would like to individually simulate in the editor.
+- **Simulated group:** This will control which available groups you would like to individually simulate in the editor. 
+
+    **Note:** A **Simulated Group must be selected** in order to create content for the **Customer Group Slot** component for that Customer Group.
 
     By default, the **No group** selection is included, which includes target customers that are not logged in or have “No group” selected as their BigCommerce group.
 
@@ -105,4 +107,4 @@ And finally, Peter’s account which doesn’t belong to a group.
 
 <Video src={"https://storage.googleapis.com/bigcommerce-production-dev-center/videos/catalyst/customer-group-component/no-group-storefront.mp4"} />
 
-We can see that each account has its own rendering of the component! Now you know how to build a section of your store using the Customer Group Slot. 
+We can see that each account has its own rendering of the component! Now you know how to build a section of your store using the Customer Group Slot.