DEVDOCS-5274:[update] GQL Overview, clarify no multiple sign in suppo…

…rt (#10)

<!-- Ticket number or summary of work -->
# [DEVDOCS-5274]


## What changed?
Document the fact that a customer can only be logged into one device at
a time

## 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]. -->
* 

## Anything else?
Old PR: bigcommerce/dev-docs#2122

ping {names}


[DEVDOCS-5274]:
https://bigcommercecloud.atlassian.net/browse/DEVDOCS-5274?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

---------

Co-authored-by: Sarah Riehl <[email protected]>
Co-authored-by: bc-andreadao <[email protected]>

docs/api-docs/storefront/graphql/graphql-api-overview.mdx

@@ -207,7 +207,11 @@ curl 'https://store.com/graphql' -H 'authorization: Bearer TOKEN_GOES_HERE' -H '
 
 ### Customer login
 
-If you're using the GraphQL Storefront API from a browser, for example, on top of your Stencil storefront, you can use the Customer Login mutation to sign in to a customer account with an email address and a password. This will set a session cookie in the browser, which will authenticate the customer account on future requests:
+If you're using the GraphQL Storefront API from a browser, for example, on top of your Stencil storefront, you can use the Customer Login mutation to sign in to a customer account with an email address and a password. This will set a session cookie in the browser, which will authenticate the customer account on future requests.
+
+<Callout type="info">
+A customer can only sign in to their account on one device. When running the customer login mutation for a session on a new device, a customer is automatically signed out.
+</Callout>
 
 ```graphql filename="Example query: login mutation" showLineNumbers copy
 mutation Login($email: String!, $pass: String!) {