Docs: add comments to bNodeType init & free callbacks

When investigating ID user-count issues, it wasn't clear how nodes
were meant to handle user-count for ID's. Specifically that the
initfunc should _not_ increment the ID's user count.

source/blender/blenkernel/BKE_node.h

@@ -271,9 +271,20 @@ typedef struct bNodeType {
   /** Check and update if internal ID data has changed. */
   void (*group_update_func)(struct bNodeTree *ntree, struct bNode *node);
 
-  /** Initialize a new node instance of this type after creation. */
+  /**
+   * Initialize a new node instance of this type after creation.
+   *
+   * \note Assignments to `node->id` must not increment the user of the ID.
+   * This is handled by the caller of this callback.
+   */
   void (*initfunc)(struct bNodeTree *ntree, struct bNode *node);
-  /** Free the node instance. */
+  /**
+   * Free the node instance.
+   *
+   * \note Access to `node->id` must be avoided in this function as this is called
+   * while freeing #Main, the state of this ID is undefined.
+   * Higher level logic to remove the node handles the user-count.
+   */
   void (*freefunc)(struct bNode *node);
   /** Make a copy of the node instance. */
   void (*copyfunc)(struct bNodeTree *dest_ntree,

source/blender/makesdna/DNA_node_types.h

@@ -330,7 +330,11 @@ typedef struct bNode {
   int16_t custom1, custom2;
   float custom3, custom4;
 
-  /** Optional link to libdata. */
+  /**
+   * Optional link to libdata.
+   *
+   * \see #bNodeType::initfunc & #bNodeType::freefunc for details on ID user-count.
+   */
   struct ID *id;
 
   /** Custom data struct for node properties for storage in files. */