docs: Clarify enum functions return value

[CodeYan01]

Description

Adds info on what the enum callbacks should return.

Motivation and Context

I was using obs_scene_enum_items but I had no idea whether I should return true or false, and the few occurrences of it in the repo only returned true, so I had to check the source code of the function for clarification.

How Has This Been Tested?

Checked the source code of the functions to see if returning false will end enumeration. As for the docs, I built it and viewed locally.

Types of changes

• Documentation (a change to documentation pages)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.