patch 8.0.1489: there is no easy way to get the global directory

Problem:    There is no easy way to get the global directory, esp. if some
            windows have a local directory.
Solution:   Make getcwd(-1) return the global directory. (Andy Massimino,
            closes #2606)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index be24e75..648e4e8 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -4484,10 +4484,13 @@
 		Without arguments, for the current window.
 
 		With {winnr} return the local current directory of this window
-		in the current tab page.
+		in the current tab page.  {winnr} can be the window number or
+		the |window-ID|.
+		If {winnr} is -1 return the name of the global working
+		directory.  See also |haslocaldir()|.
+
 		With {winnr} and {tabnr} return the local current directory of
 		the window in the specified tab page.
-		{winnr} can be the window number or the |window-ID|.
 		Return an empty string if the arguments are invalid.
 
 getfsize({fname})					*getfsize()*