Tweak "Request alternate branding" form content to be explicit about acceptable proof

## Context

Support ticket #28993

## Need

- Be more specific about accepting **digital** examples. For example a **form** or **webpage** to demonstrate need for alternate branding and exemption from GC branding.

- Or providing documents that justify a client's authority to use alternate branding (such as communication from TBS)

- We do not accept physical examples such as banners, signs, printed one-pagers.

## Implications

This is not on GC Articles. It is part of a form that would need to be republished, which would break our Freshdesk integration... this might be a better candidate to do after we have versioning released, or when there is extra dev capacity.