PR summary

• The PR fixes the issue[Doc]: Multipage PDF: unclear which backend supports and which does not support attach_note() #29428. This pull request clarifies the backend support for the attach_note feature in the multipage_pdf example documentation.
• The original documentation was unclear about which backend supports the attach_note feature. The use of the word "this" in the documentation created ambiguity, as it wasn't clear whether it referred to the PDF backend or the PGF backend.
• The wording in the documentation has been updated to ensure that users can quickly and easily understand the backend support for the attach_note feature without needing to infer it through multiple clicks and context clues.

PR checklist

• Closes #0000: This PR is linked to the issue regarding the unclear documentation on backend support for attach_note.
• [N/A] New and changed code is tested: Not applicable as this is a documentation-only change.
• [N/A] Plotting related features are demonstrated in an example: Not applicable as this is a documentation-only change.
• [N/A] New Features and API Changes are noted with a directive and release note: Not applicable as this is a documentation-only change.
• Documentation complies with general and docstring guidelines: The updated documentation complies with the guidelines specified in the Matplotlib documentation standards.