[mle] enhance logs for scheduled delayed message transmissions (#12848)

This commit updates the logging in `Mle::DelayedSender` to provide
clearer information about delayed message transmissions.

The `MessageAction` enum values `kMessageDelay` and
`kMessageRemoveDelayed` are renamed to `kMessageScheduleDelayedSend`
and `kMessageRemoveDelayedSend` to better reflect their purpose. The
corresponding string mappings are also updated to "Schedule tx of"
and "Remove scheduled tx of".

Additionally, a new log entry is added to `AddSchedule()` to explicitly
record the delay duration in milliseconds, making it easier to track
when the scheduled message is expected to be sent.

src/core/thread/mle.cpp

@@ -2886,11 +2886,11 @@ void Mle::LogError(MessageAction aAction, MessageType aType, Error aError)
 
 const char *Mle::MessageActionToString(MessageAction aAction)
 {
-#define MessageActionMapList(_)   \
-    _(kMessageSend, "Send")       \
-    _(kMessageReceive, "Receive") \
-    _(kMessageDelay, "Delay")     \
-    _(kMessageRemoveDelayed, "Remove Delayed")
+#define MessageActionMapList(_)                      \
+    _(kMessageSend, "Send")                          \
+    _(kMessageReceive, "Receive")                    \
+    _(kMessageScheduleDelayedSend, "Schedule tx of") \
+    _(kMessageRemoveDelayedSend, "Remove scheduled tx of")
 
     DefineEnumStringArray(MessageActionMapList);
 
@@ -3285,7 +3285,8 @@ void Mle::DelayedSender::AddSchedule(MessageType         aMessageType,
     mSchedules.Enqueue(*schedule);
     schedule = nullptr;
 
-    Log(kMessageDelay, aMessageType, aDestination);
+    Log(kMessageScheduleDelayedSend, aMessageType, aDestination);
+    LogInfo("    Will send in %lu msec", ToUlong(aDelay));
 
 exit:
     FreeMessage(schedule);
@@ -3455,7 +3456,7 @@ void Mle::DelayedSender::LogRemove(const Schedule &aSchedule)
     Header header;
 
     header.ReadFrom(aSchedule);
-    Log(kMessageRemoveDelayed, header.mMessageType, header.mDestination);
+    Log(kMessageRemoveDelayedSend, header.mMessageType, header.mDestination);
 }
 #else
 void Mle::DelayedSender::LogRemove(const Schedule &) {}

src/core/thread/mle.hpp

@@ -1475,8 +1475,8 @@ private:
     {
         kMessageSend,
         kMessageReceive,
-        kMessageDelay,
-        kMessageRemoveDelayed,
+        kMessageScheduleDelayedSend,
+        kMessageRemoveDelayedSend,
     };
 
     enum MessageType : uint8_t