doc: clarify the structure of ((int * int) * (int * int)) locations

[gasche]

This is silly but the existing documentation says "A pair of line and column numbers", which can be read as "A pair of line numbers and a pair of column numbers", suggesting the representation ((start_line, end_line), (start_col, end_col)) instead of the actual representation ((start_line, start_col), (end_line, end_col)). I actually made this mistake when I first tried to use decoded_range!