Fix overlay arrow display, make doc strings bidi-friendly.

 xdisp.c (display_line): Move the handling of overlay arrow after
 the call to find_row_edges.  (Bug#6699)
 cmds.c (Fforward_line, Fbeginning_of_line, Fend_of_line):
 editfns.c (Fline_beginning_position, Fline_end_position): State
 in the doc strings that start and end of line are in the logical order.