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)
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 1a6e561..1502fc7 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -1455,9 +1455,9 @@
 			{not available when compiled without the
 			|+file_in_path| feature}
 	This is a list of directories which will be searched when using the
-	|:cd| and |:lcd| commands, provided that the directory being searched
-	for has a relative path, not an absolute part starting with "/", "./"
-	or "../", the 'cdpath' option is not used then.
+	|:cd|, |:tcd| and |:lcd| commands, provided that the directory being
+	searched for has a relative path, not an absolute part starting with
+	"/", "./" or "../", the 'cdpath' option is not used then.
 	The 'cdpath' option's value has the same form and semantics as
 	|'path'|.  Also see |file-searching|.
 	The default value is taken from $CDPATH, with a "," prepended to look