patch 8.1.1218: cannot set a directory for a tab page

Problem:    Cannot set a directory for a tab page.
Solution:   Add the tab-local directory. (Yegappan Lakshmanan, closes #4212)

runtime/doc/usr_41.txt

@@ -766,7 +766,7 @@ System functions and manipulation of files:
 	isdirectory()		check if a directory exists
 	getfsize()		get the size of a file
 	getcwd()		get the current working directory
-	haslocaldir()		check if current window used |:lcd|
+	haslocaldir()		check if current window used |:lcd| or |:tcd|
 	tempname()		get the name of a temporary file
 	mkdir()			create a new directory
 	delete()		delete a file