Registration for an API key doesn't disclose constraints on fields #1803

Problem

From @sarayourfriend:

Registration instructions are not clear on the fields that need to be modified or be unique across all registered applications.
There are also general gramatical issues with this documentation that could be fixed with a quick proofread. Maybe move this documentation into the regular documentation site and out of the Swagger docs"

Description

The application's name should be unique across all the Openverse registered apps. Add a note to this section.