patch 8.1.0974: cannot switch from terminal window to previous tabpage

Problem:    Cannot switch from terminal window to previous tabpage.
Solution:   Make CTRL-W gT move to previous tabpage.
diff --git a/runtime/doc/terminal.txt b/runtime/doc/terminal.txt
index 0181d18..297f37a 100644
--- a/runtime/doc/terminal.txt
+++ b/runtime/doc/terminal.txt
@@ -81,6 +81,7 @@
 			evaluating an expression.
 	CTRL-W CTRL-C	ends the job, see below |t_CTRL-W_CTRL-C|
 	CTRL-W gt	go to next tabpage, same as `gt`
+	CTRL-W gT	go to previous tabpage, same as `gT`
 
 See option 'termwinkey' for specifying another key instead of CTRL-W that
 will work like CTRL-W.  However, typing 'termwinkey' twice sends 'termwinkey'