MctpEndpoint: Document behavior of MctpEndpoint::subscribe()

And important part of its contract is that at least one of the
registered callbacks is invoked at least once.

Change-Id: I6396e15396e1fa724f1037dad196f377fba20cb2
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>

src/MctpEndpoint.hpp

diff --git a/src/MctpEndpoint.hpp b/src/MctpEndpoint.hpp
index ef39f99..711d5e9 100644
--- a/src/MctpEndpoint.hpp
+++ b/src/MctpEndpoint.hpp
@@ -71,6 +71,10 @@
      * @brief Subscribe to events produced by an endpoint object across its
      *        lifecycle
      *
+     * @post The provided callbacks are registered for events, and at least one
+     *       callback will be invoked at least once (to guarantee visibility of
+     *       the endpoint state at the time of registration).
+     *
      * @param degraded The callback to execute when the MCTP layer indicates the
      *                 endpoint is unresponsive
      *