patch 8.1.0049: shell cannot tell running in a terminal window

Problem:    Shell cannot tell running in a terminal window.
Solution:   Add the VIM_TERMINAL environment variable. (Christian Brabandt)
diff --git a/runtime/doc/terminal.txt b/runtime/doc/terminal.txt
index b3fe522..e4cca34 100644
--- a/runtime/doc/terminal.txt
+++ b/runtime/doc/terminal.txt
@@ -376,6 +376,7 @@
     COLUMNS		number of columns in the terminal initially
     COLORS		number of colors, 't_Co' (256*256*256 in the GUI)
     VIM_SERVERNAME	v:servername
+    VIM_TERMINAL	v:version
 
 
 MS-Windows ~