HaikuBook: fix part of the warnings marked by Doxygen

I spotted some warnings when I upgraded from Doxygen 1.18.13 to 1.18.17.
The new warnings are useful, they point out imbalances in grouping, as
well as unclosed comment blocks. Coincidentally, this fixes #13338

docs/user/drivers/fs_interface.dox

@@ -1296,6 +1296,8 @@
 	\name Directory Operations
 */
 
+//! @{
+
 /*!
 	\fn status_t (*fs_vnode_ops::create_dir)(fs_volume *volume,
 			fs_vnode *parent, const char *name, int perms)
@@ -1683,7 +1685,7 @@
 	\name Node and FS Layers
 */
 
-//! {@
+//! @{
 
 /*!
 	\fn status_t (*fs_vnode_ops::create_special_node)(fs_volume *volume,
@@ -1704,6 +1706,12 @@
 
 ///// Vnode functions /////
 
+/*!
+	\name Vnode functions
+*/
+
+//! @{
+
 /*!
 	\fn status_t new_vnode(fs_volume *volume, ino_t vnodeID, void *privateNode,
 			fs_vnode_ops *ops)
@@ -1870,6 +1878,7 @@
 	\return The volume object for the given vnode.
 */
 
+//! @}
 
 ///// Notification Functions
 
@@ -1884,6 +1893,8 @@
 	that changed.
 */
 
+//! @{
+
 /*!
 	\fn status_t notify_entry_created(dev_t device, ino_t directory,
 			const char *name, ino_t node)