Returns the mark that represents the selection bound. Equivalent to calling k.TextBuffer.GetMark(string) to get the mark named "selection_bound", but very slightly more efficient, and involves less typing.

The currently-selected text in buffer is the region between the "selection_bound" and "insert" marks. If "selection_bound" and "insert" are in the same place, then there is no current selection. TextBuffer.GetSelectionBounds(TextIter, TextIter) is another convenient function for handling the selection, if you just want to know whether there is a selection and what its bounds are.