Dev Docs: Describe Serialized Block Header And Block Format

* Replace current description of the block header with a better
description.

    * Describe the various version numbers.

    * Describe how the merkle root is constructed.

    * Describe how nBits is parsed and how to correctly create it to
      avoid negative values.

* Describe the serialized block format used to calculate max block size.
Dev Docs: Merkle Tree/nBits Minor Proofreading Edits

Based on feedback from @saivann (thanks!)
Merge branch 'serialized-blocks' (pull #638)