docs: Explain what the "advanced regex" does (#755)

Author: mayank23

docs/queries/about.mdx

@@ -259,7 +259,7 @@ screen.getByText('hello world', { exact: false }) // ignore case
 screen.getByText(/World/) // substring match
 screen.getByText(/world/i) // substring match, ignore case
 screen.getByText(/^hello world$/i) // full string match, ignore case
-screen.getByText(/Hello W?oRlD/i) // advanced regex
+screen.getByText(/Hello W?oRlD/i) // substring match, ignore case, searches for "hello world" or "hello orld"
 
 // Matching with a custom function:
 screen.getByText((content, element) => content.startsWith('Hello'))