Explain the range (u8), and the special case for ID 0.

The endpoints here are for all the connections, while the ID 0 is a
special case per connection, depending on the source address used by the
initial subflow. This ID 0 can then not be used for the global
endpoints.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 man/man8/ip-mptcp.8 | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/man/man8/ip-mptcp.8 b/man/man8/ip-mptcp.8
index 7eeeeef2..9c2e4ea6 100644
--- a/man/man8/ip-mptcp.8
+++ b/man/man8/ip-mptcp.8
@@ -148,7 +148,11 @@ expected behavior.
 
 .TP
 .IR ID
-is a unique numeric identifier for the given endpoint
+is a unique numeric identifier, between 0 and 255, for the given endpoint. It is
+not possible to add endpoints with ID 0, because this special ID is reserved for
+the initial subflow. For rules linked to the initial subflow, the path-manager
+will look at endpoints matching the same address, and port if set, ignoring the
+ID.
 
 .TP
 .BR signal

-- 
2.45.2