Restore and document additional `return_input` args for `overlap`

[nvictus]

Values besides True/False were accidentally disabled in a refactor and never got documented (or tested).

return_input:

• True -- return both left and right
• False -- don't return original input (either side)
• 1, "1", "left" -- return left only
• 2, "2", "right" -- return right only