Motivation

At the moment it is not clear whether by setting exclude to my own list, I am disabling the default exclusions.
If exclude replaces the default exclusions, it may be that I do not need some of the includes I added for file which were excluded by the default exclusions. Or if I need to repeat those default exclusions to not let undesired files into my build.
If exclude appends to the default excludes, the behavior is IMO more intuitive and desirable, but it should be commented so that a user can be sure that is the case.

Suggestion

Expand the Exclude row in https://jekyllrb.com/docs/configuration/options.
For Exclude, either mention it is added to the default exclusions or replaces them.
Also mention the default exclusions. I know dotfiles are excluded (from the Include row), what else? Is there documentation for this elsewhere that I missed?

I am happy to PR this myself, but I am lacking the information. Can anyone help me determine which is the case with exclude and what are the default exclusions?