patch 8.1.0870: Vim doesn't use the new ConPTY support in Windows 10

Problem:    Vim doesn't use the new ConPTY support in Windows 10.
Solution:   Use ConPTY support, if available. (Nobuhiro Takasaki, closes #3794)

runtime/doc/terminal.txt

@@ -228,7 +228,8 @@ Syntax ~
 					for Python "++eof=exit()".  Special
 					codes can be used like with `:map`,
 					e.g. "<C-Z>" for CTRL-Z.
-
+			++winpty	Use winpty as the virtual console.
+			++conpty	Use |ConPTY| as the virtual console.
 			If you want to use more options use the |term_start()|
 			function.
 			If you want to split the window vertically, use: >
@@ -410,6 +411,13 @@ Just put the files somewhere in your PATH.  You can set the 'winptydll' option
 to point to the right file, if needed.  If you have both the 32-bit and 64-bit
 version, rename to winpty32.dll and winpty64.dll to match the way Vim was
 build.
+							*ConPTY*
+On more recent versions of MS-Windows 10 (beginning with the "October 2018
+Update"), winpty is no longer required. On those versions, |:terminal| will use
+Windows' built-in support for hosting terminal applications, "ConPTY".  When
+ConPTY is in use, there may be rendering artifacts regarding ambiguous-width
+characters. If you encounter any such issues, set 'termmode' to winpty (which
+you then must have instlled).
 
 Environment variables are used to pass information to the running job:
     VIM_SERVERNAME	v:servername