194e233d86 caused a discussion in the chat about the initialization
behavior of `MEM_new()`, and agreement was to not rely on
zero-initialization ever. Noted this in the API comment now.
Some people found the existing comment useful but it still left some
questions. Tried to clarify that now.
This is a crucial memory management function, it's important to have
behavior documented well, even if a full explanation is out-of-scope.
Also added another link in case people want to check more details.
Pull Request: https://projects.blender.org/blender/blender/pulls/134577
152c6c54e4