patch 8.1.0909: MS-Windows: using ConPTY even though it is not stable

Problem:    MS-Windows: using ConPTY even though it is not stable.
Solution:   When ConPTY version is unstable, prefer using winpty. (Ken Takata,
            closes #3949)
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index 4133420..6b2dde8 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -8112,12 +8112,14 @@
 	window.
 
 	Possible values are:
-	    ""		use ConPTY if possible, winpty otherwise
+	    ""		use ConPTY if it is stable, winpty otherwise
 	    "winpty"	use winpty, fail if not supported
 	    "conpty"	use |ConPTY|, fail if not supported
 
-	|ConPTY| support depends on the platform (Windows 10 October 2018
-	edition).  winpty support needs to be installed.  If neither is
+	|ConPTY| support depends on the platform.  Windows 10 October 2018
+	Update is the first version that supports ConPTY, however it is still
+	considered unstable.  ConPTY might become stable in the next release
+	of Windows 10.  winpty support needs to be installed.  If neither is
 	supported then you cannot open a terminal window.
 
 						*'terse'* *'noterse'*