Add missing activation documentation for FETA

[timokau]

Description

I missed documentation for the activation parameter when adding the FETA based estimators in #183.

Motivation and Context

See description.

How Has This Been Tested?

I ran the linters.

Does this close/impact existing issues?

No issues, but this is a follow up to #183.

Types of changes

"Bug fix" is the closest match. This does not affect the functionality though.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.