doc: networking: ip-sysctl: Document conf/all/disable_ipv6 and conf/default/disable_ipv6

This patch adds documentation for sysctl conf/all/disable_ipv6 and
conf/default/disable_ipv6 settings which is currently missing.
Signed-off-by: Pali Rohár <pali@kernel.org>
Link: https://lore.kernel.org/r/20210121150244.20483-1-pali@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/ip-sysctl.rst

@@ -1807,12 +1807,24 @@ seg6_flowlabel - INTEGER
 ``conf/default/*``:
 	Change the interface-specific default settings.
 
+	These settings would be used during creating new interfaces.
+
 
 ``conf/all/*``:
 	Change all the interface-specific settings.
 
 	[XXX:  Other special features than forwarding?]
 
+conf/all/disable_ipv6 - BOOLEAN
+	Changing this value is same as changing ``conf/default/disable_ipv6``
+	setting and also all per-interface ``disable_ipv6`` settings to the same
+	value.
+
+	Reading this value does not have any particular meaning. It does not say
+	whether IPv6 support is enabled or disabled. Returned value can be 1
+	also in the case when some interface has ``disable_ipv6`` set to 0 and
+	has configured IPv6 addresses.
+
 conf/all/forwarding - BOOLEAN
 	Enable global IPv6 forwarding between all interfaces.