patch 8.1.0929: no error when requesting ConPTY but it's not available

Problem:    No error when requesting ConPTY but it's not available.
Solution:   Add an error message. (Hirohito Higashi, closes #3967)
diff --git a/runtime/doc/terminal.txt b/runtime/doc/terminal.txt
index 4b009d3..42fe2c9 100644
--- a/runtime/doc/terminal.txt
+++ b/runtime/doc/terminal.txt
@@ -413,13 +413,13 @@
 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*
+							*ConPTY* *E982*
 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 'termwintype' to "winpty"
-(which you then must have instlled).
+characters. If you encounter any such issues, install "winpty".  Until the
+ConPTY problems have been fixed "winpty" will be preferred.
 
 Environment variables are used to pass information to the running job:
     VIM_SERVERNAME	v:servername