Add note about defining custom environments in a separate module

[bcrawford39GT]

To avoid warnings when adding/using environments.

Description

Added some text in the documentation on how to add environments without getting warnings.

Motivation and Context

Checklist:

• [ x] I am familiar with the Contributing Guidelines.
• [x ] I agree with the terms of the Contributor Agreement.
• [x ] My code follows the code style guideline of this project.

[bcrawford39GT]

completed changes