Document hs dht min uptime change in privnets

In f87c6f100d2faf8d we made it so that MinUptimeHidServDirectoryV2
defaults to 0 if TestingTorNetwork is set. Add a manpage entry and a
changes file.

changes/minuptimehs

@@ -0,0 +1,3 @@
+  o Minor features:
+    - Make hidden services work better in private networks by not requiring
+      any uptime to join the dht. Implements ticket 2088.

doc/tor.1.txt

@@ -1267,6 +1267,7 @@ The following options are used for running a testing Tor network.
        V3AuthVotingInterval 5 minutes
        V3AuthVoteDelay 20 seconds
        V3AuthDistDelay 20 seconds
+       MinUptimeHidServDirectoryV2 0 seconds
        TestingV3AuthInitialVotingInterval 5 minutes
        TestingV3AuthInitialVoteDelay 20 seconds
        TestingV3AuthInitialDistDelay 20 seconds