Update newContext to reflect 1-to-1 change

The discription on browser.newContext needs to reflect the new change
where only one browserContext can be open at any given time per
browser. We need to notify the user how they can work with this new
behaviour.

Author: ankur22

src/data/markdown/docs/02 javascript api/07 k6-experimental/01 browser/newcontext--options--.md

@@ -3,7 +3,13 @@ title: 'newContext([options])'
 excerpt: 'Browser module: newContext method'
 ---
 
-Creates and returns a new [BrowserContext](/javascript-api/k6-experimental/browser/browsercontext/).
+Creates and returns a new [BrowserContext](/javascript-api/k6-experimental/browser/browsercontext/), if one hasn't already been initialized for the [Browser](/javascript-api/k6-experimental/browser). If one has already been initialized an error is thrown.
+
+<Blockquote mod="note" title="">
+
+A 1-to-1 mapping between [Browser](/javascript-api/k6-experimental/browser) and `BrowserContext` means you cannot run `BrowserContexts` concurrently. Due to this restriction, if one already exists, it must be [close](/javascript-api/k6-experimental/browser/browsercontext/close)d first before creating a new one.
+
+</Blockquote>
 
 <TableWithNestedRows>