crypto: Document the meaning of compression level zero
crypto: Document the meaning of compression level zero

- Document that passing a zero compression level to isrcry_compress_init()
will use the algorithm's default level

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr