Document the new structures better

lib/include/pdb_int_n.h

 #include "pdb_conf.h"
 
 
+/*
+ * Structure for the INT_N thread
+ */
 struct pdb_int_n {
+    /* INT_N thread and working area */
     THD_WORKING_AREA(_wa, PDB_INT_N_WA_SIZE);
     thread_t *thread;
 };

lib/include/pdb_pe.h

  * Structure for Policy Engine thread and variables
  */
 struct pdb_pe {
+    /* Policy Engine thread and working area */
     THD_WORKING_AREA(_wa, PDB_PE_WA_SIZE);
     thread_t *thread;
 

lib/include/pdb_prl.h

 #include "pdb_conf.h"
 
 
+/*
+ * Structure for the protocol layer threads and variables
+ */
 struct pdb_prl {
+    /* RX thread and working area */
     THD_WORKING_AREA(_rx_wa, PDB_PRLRX_WA_SIZE);
     thread_t *rx_thread;
+    /* TX thread and working area */
     THD_WORKING_AREA(_tx_wa, PDB_PRLTX_WA_SIZE);
     thread_t *tx_thread;
+    /* Hard reset thread and working area */
     THD_WORKING_AREA(_hardrst_wa, PDB_HARDRST_WA_SIZE);
     thread_t *hardrst_thread;
 
+    /* TX mailbox for PD messages to be transmitted */
     mailbox_t tx_mailbox;
 
+    /* The ID of the last message received */
     int8_t _rx_messageid;
+    /* The message being worked with by the RX thread */
     union pd_msg *_rx_message;
 
+    /* The ID of the next message we will transmit */
     int8_t _tx_messageidcounter;
+    /* The message being worked with by the TX thread */
     union pd_msg *_tx_message;
+    /* Queue for the TX mailbox */
     msg_t _tx_mailbox_queue[PDB_MSG_POOL_SIZE];
 };