Clarify that door passing is a special case of descriptor passing

[robertdfrench]

From u/jking13:

When you talk about passing a door through a door, it might be good to note that you're just using the feature that allows doors to pass file descriptors, just the descriptor in that case happens to be a door -- it could be a file, a socket, a pipe, etc.

This begs the question of whether there are other general means for passing descriptors to another process, and whether one could pass a door descriptor using those. If so it would be good to have a contrasting example so that I do not paint doors as more special than they really are.