doc: mptcp: sysctl: blackhole_timeout is per-netns

All other sysctl entries mention it, and it is a per-namespace sysctl.

So mention it as well.

Fixes: 27069e7 ("mptcp: disable active MPTCP in case of blackhole")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/networking/mptcp-sysctl.rst

@@ -41,7 +41,7 @@ blackhole_timeout - INTEGER (seconds)
 	MPTCP is re-enabled and will reset to the initial value when the
 	blackhole issue goes away.
 
-	0 to disable the blackhole detection.
+	0 to disable the blackhole detection. This is a per-namespace sysctl.
 
 	Default: 3600