docs: fix block.Header.Time description (#3529)

It's not proposer local time anymore, but a weighted median

Fixes #3514

CHANGELOG_PENDING.md

@@ -20,3 +20,4 @@
 
 ### BUG FIXES:
 - [p2p] \#2716 Check if we're already connected to peer right before dialing it (@melekes)
+- [docs] \#3514 Fix block.Header.Time description (@melekes)

docs/spec/abci/abci.md

@@ -347,8 +347,10 @@ Commit are included in the header of the next block.
   - `Version (Version)`: Version of the blockchain and the application
   - `ChainID (string)`: ID of the blockchain
   - `Height (int64)`: Height of the block in the chain
-  - `Time (google.protobuf.Timestamp)`: Time of the block. It is the proposer's
-    local time when block was created.
+  - `Time (google.protobuf.Timestamp)`: Time of the previous block.
+    For heights > 1, it's the weighted median of the timestamps of the valid
+    votes in the block.LastCommit.
+    For height == 1, it's genesis time.
   - `NumTxs (int32)`: Number of transactions in the block
   - `TotalTxs (int64)`: Total number of transactions in the blockchain until
     now

docs/spec/blockchain/blockchain.md

@@ -244,7 +244,7 @@ The height is an incrementing integer. The first block has `block.Header.Height
 ### Time
 
 ```
-block.Header.Timestamp >= prevBlock.Header.Timestamp + 1 ms
+block.Header.Timestamp >= prevBlock.Header.Timestamp + state.consensusParams.Block.TimeIotaMs
 block.Header.Timestamp == MedianTime(block.LastCommit, state.LastValidators)
 ```