Update document upload help text

[srancour]

User Story or Problem Statement

As an editor I need to know how to make my PDFs accessible while uploading my documents.

Description or Additional Context

The help text on the Document settings for Document page needs to be changed from For PDFs, please make sure the text can be cleanly copied out of the file – simple formatting is best. to For PDFs, please make sure that it has been made to be accessible. Accessibility standards for PDFs can be accessed in the [VA Directive 6102 KB article's Accessibility section](https://prod.cms.va.gov/help/va-directive-6102#accessibility).

Acceptance Criteria

• Help text has been changed and the link to the KB article is included.