Document that an IPv6 nserver address needs square brackets

nserver and authnserver parse their argument with parsehost(), which reads
the first colon as the address/port separator, so a bare IPv6 address picks
up a bogus port number. Document the bracket form in 3proxy.cfg(5) and in
both HOWTOs.

Refs #1275, #460

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

doc/html/howtoe.html

@@ -1942,10 +1942,12 @@ allowed traffic in megabytes (MB). nocountin allows you to set exclusions.
  For name resolution and caching, use the commands nserver, nscache / nscache6, and nsrecord.
   <pre>
   nserver 192.168.1.2
-  nserver 192.168.1.3:5353/tcp</pre>
+  nserver 192.168.1.3:5353/tcp
+  nserver [2001:4860:4860::8844]</pre>
  sets DNS resolvers. 192.168.1.3 will be used via TCP/5353 (instead of default UDP/53)
  only if 192.168.1.2 fails. Up to 5 nservers may be specified.
  If no nserver is configured, default system name resolution functions are used.
+ An IPv6 address has to be written in square brackets.
   <pre>
   nscache 65535
   nscache6 65535</pre>

doc/html/howtor.html

@@ -1993,11 +1993,13 @@ socks -p1080
  nscache и nsrecord.
   <pre>
   nserver 192.168.1.2
-  nserver 192.168.1.3:5353/tcp</pre>
+  nserver 192.168.1.3:5353/tcp
+  nserver [2001:4860:4860::8844]</pre>
  указывает 3proxy какие машины следует использвоать в качестве серверов
  DNS. Сервер 192.168.1.3 будет использоваться по порту TCP/5353 (вместо дефолтного UDP/53) только при недостижимости
  192.168.1.2. Можно указать до 5 серверов. Если nserver не указан, будут
  использованы системные функции разрешения имен.
+ Адрес IPv6 необходимо записывать в квадратных скобках.
   <pre>
   nscache 65535
   nscache6 65535</pre>

doc/html/man5/3proxy.cfg.5.html

@@ -482,7 +482,11 @@ experimental.</p>
 Nameserver to use for name resolutions. If none specified
 system routines for name resolution is used. Optional port
 number may be specified. If optional /tcp is added to IP
-address, name resolution is performed over TCP.</p>
+address, name resolution is performed over TCP. An IPv6 address
+has to be enclosed in square brackets: <br>
+<b>nserver 1.1.1.1</b> <br>
+<b>nserver [2001:4860:4860::8844]</b> <br>
+<b>nserver [2001:4860:4860::8844]:5353/tcp</b></p>
 
 
 <p style="margin-left:9%; margin-top: 1em"><b>authnserver</b>

man/3proxy.cfg.5

@@ -517,6 +517,13 @@ system routines for name resolution is
 used. Optional port number may be specified.
 If optional /tcp is added to IP address, name resolution is
 performed over TCP.
+An IPv6 address has to be enclosed in square brackets:
+.br
+ \fBnserver 1.1.1.1\fR
+.br
+ \fBnserver [2001:4860:4860::8844]\fR
+.br
+ \fBnserver [2001:4860:4860::8844]:5353/tcp\fR
 
 .br
 .BR authnserver