patch 8.2.0970: terminal properties are not available in Vim script

Problem:    Terminal properties are not available in Vim script.
Solution:   Add the terminalprops() function.
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index e39f1fe..0219b98 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -1148,6 +1148,7 @@
 	getimstatus()		check if IME status is active
 	interrupt()		interrupt script execution
 	windowsversion()	get MS-Windows version
+	terminalprops()		properties of the terminal
 
 	libcall()		call a function in an external library
 	libcallnr()		idem, returning a number