docs(mdx): note that getHeadings() excludes headings from imported files

[SRKrukowski]

Closes #14042.

On the @astrojs/mdx page, the getHeadings() exported property doesn't mention that headings from imported MDX sub-files are not included in its output. This surprised the issue reporter.

Per @ArmandPhilippot's suggestion in the issue (and confirmed to be MDX-specific), this appends a single sentence to the existing description rather than adding an aside:

...can be used for anchor links. Headings from imported files are not included.

Docs-only, one line.

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