Fix #256: <RIGHT> for insert will go to line end.

The default "move right accross lines" goes from before / on the last
character to the next line, but in insert mode our caret can be after
that char. Keep these cases apart through the new INSTANCE_INSERT
so that "move right across lines" in insert mode goes one position
further.

Revert stick column code, check `updateStickyColumn()` instead.
Update documentation about insert remaps for #256

Not all special keys are supported in remaps for insert mode. They are
now marked in the documentation.