-must exist already. If the file is smaller than C<size> then
-the file is extended to the required size with null bytes.");
+must exist already.
+
+If the current file size is less than C<size> then
+the file is extended to the required size with zero bytes.
+This creates a sparse file (ie. disk blocks are not allocated
+for the file until you write to it). To create a non-sparse
+file of zeroes, use C<guestfs_fallocate64> instead.");