docs: add note about MAC addresses not being unique

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 763aceeb733cdd927eb2de6232f0257267b6f71e)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/run.md

@@ -291,7 +291,8 @@ you can override this with `--dns`.
 
 By default, the MAC address is generated using the IP address allocated to the
 container. You can set the container's MAC address explicitly by providing a
-MAC address via the `--mac-address` parameter (format:`12:34:56:78:9a:bc`).
+MAC address via the `--mac-address` parameter (format:`12:34:56:78:9a:bc`).Be
+aware that Docker does not check if manually specified MAC addresses are unique.
 
 Supported networks :