Clarify comment about all tor data being encrypted

This isn't quite accurate:

  /* Don't actually allow compression; it uses ram and time, but the data
   * we transmit is all encrypted anyway. */

The following "data" isn't encrypted:

• cell headers
• non-relay cell types

I suggest:

  /* Don't actually allow compression; it uses ram and time, but the
   * circuit data we transmit is encrypted anyway. */