Support lower bound on hscrolling when only current line scrolls

Support lower bound on hscrolling when only current line scrolls

* doc/emacs/display.texi (Horizontal Scrolling): Document the new
mode of auto-hscrolling only the current line.

* src/xdisp.c (init_iterator): When hscrolling only the
current line, apply the	window's min_hscroll here, so that
non-current lines will be hscrolled by that minimum.
Suggested by Stephen Berman <stephen.berman@gmx.net>.
(hscroll_window_tree): Account for window's min_hscroll when
deciding whether to recompute the hscroll.
(display_line): Subtract window's min_hscroll from x_incr, as that
was already accounted for in init_iterator.  (Bug#27008)