patch 8.2.0557: no IPv6 support for channels

Problem:    No IPv6 support for channels.
Solution:   Add IPv6 support. (Ozaki Kiichi, closes #5893)
diff --git a/runtime/doc/various.txt b/runtime/doc/various.txt
index b6867bc..a88cc81 100644
--- a/runtime/doc/various.txt
+++ b/runtime/doc/various.txt
@@ -375,6 +375,7 @@
    *+iconv*		Compiled with the |iconv()| function
    *+iconv/dyn*		Likewise |iconv-dynamic| |/dyn|
 T  *+insert_expand*	|insert_expand| Insert mode completion
+m  *+ipv6*		Support for IPv6 networking |channel|
 m  *+job*		starting and stopping jobs |job|
 S  *+jumplist*		|jumplist|
 B  *+keymap*		|'keymap'|