Describe interaction of getHeadings with MDX imports (#14072)

Co-authored-by: Armand Philippot <59021693+ArmandPhilippot@users.noreply.github.com>

Author: TimonLukas

src/content/docs/en/guides/integrations-guide/mdx.mdx

@@ -147,7 +147,7 @@ The following properties are available to a `.astro` component when using an `im
 - **`file`** - The absolute file path (e.g. `/home/user/projects/.../file.mdx`).
 - **`url`** - The URL of the page (e.g. `/en/guides/markdown-content`).
 - **`frontmatter`** - Contains any data specified in the file’s YAML/TOML frontmatter.
-- **`getHeadings()`** - An async function that returns an array of all headings (`<h1>` to `<h6>`) in the file with the type: `{ depth: number; slug: string; text: string }[]`. Each heading’s `slug` corresponds to the generated ID for a given heading and can be used for anchor links.
+- **`getHeadings()`** - An async function that returns an array of all headings (`<h1>` to `<h6>`) in the file with the type: `{ depth: number; slug: string; text: string }[]`. Each heading’s `slug` corresponds to the generated ID for a given heading and can be used for anchor links. Headings from imported files are not included.
 - **`<Content />`** - A component that returns the full, rendered contents of the file.
 - **(any `export` value)** - MDX files can also export data with an `export` statement.