prop224: Clarify descriptor upload section.

   approximately 21 hours and 38 minutes after the beginning of the

   period.

   The new shared random value MUST be published *before* the overlap interval

   New shared random values MUST be published *before* the overlap interval

   starts so that hidden services have access to the new shared random values

   in time and can calculate the upcoming set of responsible HSDirs. In our

   system, new shared random values get published at 00:00UTC every day, whereas

   the overlap period starts at 06:00 and finishes at 12:00UTC.

   system, new shared random values get published at 00:00UTC every day,

   whereas the overlap period starts at 06:00 and finishes at 12:00UTC.

   Here is an illustration of the system:

            hs_index(replicanum) = H("store-at-idx" |

                                 blinded_public_key |

                                 INT_8(replicanum) |

                                 INT_8(periodnum) )

   where blinded_public_key is specified in section KEYBLIND, and

   periodnum is defined in section TIME-PERIODS.

                                 INT_8(period_num) )

   where n_replicas is determined by the consensus parameter

   "hsdir_n_replicas".

   where blinded_public_key is specified in section KEYBLIND, and period_num is

   defined in section [TIME-PERIODS].

   Then, for each node listed in the current consensus with the HSDir3

   flag, we compute a directory index for that node as:

   Then, for each node listed in the current consensus with the HSDirV3 flag,

   we compute a directory index for that node as:

           hsdir_index(node) = H("node-idx" | node_identity_digest |

                                 shared_random |

                                 shared_random_value |

                                 INT_8(period_num) )

   where shared_random is the shared value generated by the authorities

   in section PUB-SHAREDRANDOM, and node_identity_digest is a SHA1

   digest of the node's RSA public key as described in tor-spec.txt.

   where shared_random_value is the shared value generated by the authorities

   in section [PUB-SHAREDRANDOM], and node_identity_digest is a SHA1 digest of

   the node's RSA public key as described in tor-spec.txt.

   Finally, for replicanum in 1...hsdir_n_replicas, the hidden service

   host uploads descriptors to the first hsdir_spread_store nodes whose

   Again, nodes from lower-numbered replicas are disregarded when

   choosing the spread for a replica.

   Hidden services MUST periodically re-publish their descriptor to the

   responsible HSDirs. Specifically, hidden services re-publish their

   descriptors every 1 hour (also controlled via the 'hs_rend_post_period'

   consensus parameter).

   HSDirs MUST retain hidden service descriptors for 33 hours before expiring

   them. That's 24 hours for the time period duration, plus 6 hours for the

   maximum overlap period span, plus 3 hours for the maximum acceptable client