patch 9.0.1710: scrolloff options work slightly different

Problem: sidescrolloff and scrolloff options work slightly
         different than other global-local options
Solution: Make it behave consistent for all global-local options

It was noticed, that sidescrolloff and scrolloff options behave
differently in comparison to other global-local window options like
'listchars'

So make those two behave like other global-local options. Also add some
extra documentation for a few special local-window options.

Add a few tests to make sure all global-local window options behave
similar

closes: #12956
closes: #12643

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/window.c b/src/window.c
index f02aa9f..33d19fa 100644
--- a/src/window.c
+++ b/src/window.c
@@ -5546,8 +5546,8 @@
     new_wp->w_scbind_pos = 1;
 
     // use global option value for global-local options
-    new_wp->w_p_so = -1;
-    new_wp->w_p_siso = -1;
+    new_wp->w_allbuf_opt.wo_so = new_wp->w_p_so = -1;
+    new_wp->w_allbuf_opt.wo_siso = new_wp->w_p_siso = -1;
 
     // We won't calculate w_fraction until resizing the window
     new_wp->w_fraction = 0;