doc: improvement to IPC documentation

Per default, doxygen will take only the part until the first dot into
account for the brief description. In this case this leads to an
ambiguous overview over the IPC send functions.

core/include/msg.h

@@ -60,7 +60,7 @@ typedef struct msg {
 
 
 /**
- * @brief Send a message. (blocking)
+ * @brief Send a message (blocking).
  *
  * This function sends a message to another thread. The ``msg_t`` structure has
  * to be allocated (e.g. on the stack) before calling the function and can be
@@ -81,7 +81,7 @@ int msg_send(msg_t *m, kernel_pid_t target_pid);
 
 
 /**
- * @brief Send a message. (non-blocking)
+ * @brief Send a message (non-blocking).
  *
  * This function sends a message to another thread. The ``msg_t`` structure has
  * to be allocated (e.g. on the stack) before calling the function and can be