addLink: Bugfix and Quality-of-Life Improvements

All changes in `pdf.PdfFileWriter.addLink`:
Added documentation to make use of this method a bit clearer.
Fixed an issue where `/P` was mapped to a repr'd `IndirectObject` rather than a proper indirect reference.
Added support for a border-styling argument (as a list).
Allowed `rect` to be a string (as before), or a `RectangleObject`, or a `list`, to make use more intuitive.
Finally, removed the mysterious magic `NumberObject(826)` that was hardcoded into the `/Dest` (does anyone know why this was in there?)