Improve documentation of line truncation and wrapping

Improve documentation of line truncation and wrapping

* doc/emacs/display.texi (Line Truncation, Visual Line Mode):
* doc/lispref/display.texi (Truncation): Document that turning on
line truncation disables wrapping, and vice versa.

* src/buffer.c (syms_of_buffer) <truncate-lines>:
* src/xdisp.c (syms_of_xdisp) <truncate-partial-width-windows>:
Warn against turning on when 'visual-line-mode' is in effect.
* lisp/simple.el (visual-line-mode): Document that this mode
disables line truncation.  (Bug#29664)