CH11: update para about height not being stored in blocks

ch09.asciidoc

@@ -167,10 +167,14 @@ Although a single block will always have a specific and invariant block
 height, the reverse is not true—the block height does not always
 identify a single block. Two or more blocks might have the same block
 height, competing for the same position in the blockchain. This scenario
-is discussed in detail in the section <<forks>>. The block height is
-also not a part of the block's data structure; it is not stored within
-the block. Each node dynamically identifies a block's position (height)
-in the blockchain when it is received from the Bitcoin network. The
+is discussed in detail in the section <<forks>>.  In early blocks, the block height was
+also not a part of the block's data structure; it was not stored within
+the block. Each node dynamically identified a block's position (height)
+in the blockchain when it was received from the Bitcoin network.  A
+later protocol change (BIP34) began including the block height in the
+coinbase transaction, although it's purpose was to ensure each block had
+a different coinbase transaction.  Nodes still need to dynamically
+identify a block's height in order to validate the coinbase field.  The
 block height might also be stored as metadata in an indexed database
 table for faster retrieval.