Improve documentation of set-window-start

* doc/lispref/windows.texi (Window Start and End):
* src/window.c (Fset_window_start): Document that reliable
setting of a window start position requires to adjust point to
be visible.  (Bug#34038)
1 job for emacs-26 in 26 minutes and 24 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #1240
test

00:26:24