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/channel.txt b/runtime/doc/channel.txt
index 3fa0508..42e00c7 100644
--- a/runtime/doc/channel.txt
+++ b/runtime/doc/channel.txt
@@ -120,6 +120,9 @@
 
 {address} has the form "hostname:port".  E.g., "localhost:8765".
 
+When using an IPv6 address, enclose it within square brackets.  E.g.,
+"[2001:db8::1]:8765".
+
 {options} is a dictionary with optional entries:	*channel-open-options*
 
 "mode" can be:						*channel-mode*
@@ -621,6 +624,9 @@
 		{address} has the form "hostname:port", e.g.,
 		"localhost:8765".
 
+		When using an IPv6 address, enclose it within square brackets.
+		E.g., "[2001:db8::1]:8765".
+
 		If {options} is given it must be a |Dictionary|.
 		See |channel-open-options|.
 
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'|