patch 8.2.3627: difficult to know where the text starts in a window

Problem:    difficult to know where the text starts in a window. (Sergey
            Vlasov)
Solution:   Add the "textoff" entry in the result of getwininfo().
            (closes #9163)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 593dcb5..1003010 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -6382,6 +6382,9 @@
 					otherwise
 			wincol		leftmost screen column of the window;
 					"col" from |win_screenpos()|
+			textoff		number of columns occupied by any
+					'foldcolumn', 'signcolumn' and line
+					number in front of the text
 			winid		|window-ID|
 			winnr		window number
 			winrow		topmost screen line of the window;