Fix incorrect block metadata documentation (#378)

The documentation speaks of a block time as part of the block
metadata, which is not true.

I fixed it and also slightly made this more up to date.

Change-Id: Idaa6c26ba205a9c3c3300f2cdf10489acbcdd12e
Signed-off-by: yacovm <yacovm@il.ibm.com>

docs/source/ledger/ledger.md

@@ -234,11 +234,13 @@ sections
 
 * **Block Metadata**
 
-  This section contains the time when the block was written, as well as the
-  certificate, public key and signature of the block writer. Subsequently, the
-  block committer also adds a valid/invalid indicator for every transaction,
-  though this information is not included in the hash, as that is created when
-  the block is created.
+  This section contains the certificate and signature of the block creator which is used to verify
+  the block by network nodes.
+  Subsequently, the block committer adds a valid/invalid indicator for every transaction into
+  a bitmap that also resides in the block metadata, as well as a hash of the cumulative state updates
+  up until and including that block, in order to detect a state fork.
+  Unlike the block data  and header fields, this section is not an input to the block hash computation.
+
 
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