drm/ttm: Improve comment in ttm_bo_vm_ctor() about lack of ref acquisition

Approved by:	kib@

sys/dev/drm2/ttm/ttm_bo_vm.c

@@ -285,8 +285,16 @@ ttm_bo_vm_ctor(void *handle, vm_ooffset_t size, vm_prot_t prot,
 {
 
 	/*
-	 * We don't acquire a reference on bo->kref here, because it was
-	 * already done in ttm_bo_mmap_single().
+	 * On Linux, a reference to the buffer object is acquired here.
+	 * The reason is that this function is not called when the
+	 * mmap() is initialized, but only when a process forks for
+	 * instance. Therefore on Linux, the reference on the bo is
+	 * acquired either in ttm_bo_mmap() or ttm_bo_vm_open(). It's
+	 * then released in ttm_bo_vm_close().
+	 *
+	 * Here, this function is called during mmap() intialization.
+	 * Thus, the reference acquired in ttm_bo_mmap_single() is
+	 * sufficient.
 	 */
 
 	*color = 0;