Contributions by @saivann to devel docs

Thanks also (in alphabetical order) to @cbeams, @mikehearn, and
@tgeller, among others.

The last pre-squash commit was: c2b8d562aa107c7b68c60946cea14cdccc5159ad

_includes/ref_block_chain.md

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+## Block Chain
+
+The following subsections briefly document core block details.
+
+### Block Contents
+
+{% autocrossref %}
+
+This section describes [version 2 blocks][v2 block]{:#term-v2-block}{:.term}, which are any blocks with a
+block height greater than 227,835. (Version 1 and version 2 blocks were
+intermingled for some time before that point.) Future block versions may
+break compatibility with the information in this section. You can determine
+the version of any block by checking its `version` field using
+bitcoind RPC calls.
+
+As of version 2 blocks, each block consists of four root elements:
+
+1. A [magic number][block header magic]{:#term-block-header-magic}{:.term} (0xd9b4bef9).
+
+2. A 4-byte unsigned integer indicating how many bytes follow until the
+   end of the block. Although this field would suggest maximum block
+   sizes of 4 GiB, max block size is currently capped at 1 MiB and the
+   default max block size (used by most miners) is 350 KiB (although
+   this will likely increase over time).
+
+3. An 80-byte block header described in the section below.
+
+4. One or more transactions.
+
+Every block must include a [coinbase transaction][]{:#term-coinbase-tx}{:.term} which should collect and
+spend any transaction fees paid by transactions included in this block.
+All blocks with a block height less than 6,930,000 are entitled to
+receive a [block reward][]{:#term-block-reward}{:.term} of newly created bitcoin value, which also
+should be spent in the coinbase transaction. (The block reward started
+at 50 bitcoins and is being halved approximately every four years. As of
+April 2014, it's 25 bitcoins.) A coinbase transaction is invalid if it 
+tries to spend more value than is available from the transaction 
+fees and block reward.
+
+The coinbase transaction has the same basic format as any other
+transaction, but it references a single non-existent UTXO and a special
+[coinbase field][]{:#term-coinbase-field}{:.term} replaces the field that would normally hold a scriptSig and
+signature. In version 2 blocks, the coinbase parameter must begin with
+the current block's block height and may contain additional arbitrary
+data or a script up to a maximum total of 100 bytes.
+
+{% endautocrossref %}
+
+### Block Header
+
+{% autocrossref %}
+
+The 80-byte block header contains the following six fields:
+
+| Field             | Bytes  | Format                         |
+|-------------------|--------|--------------------------------|
+| 1. Version        | 4      | Unsigned Int                   |
+| 2. hashPrevBlock  | 32     | Unsigned Int (SHA256 Hash)     |
+| 3. hashMerkleRoot | 32     | Unsigned Int (SHA256 Hash)     |
+| 4. Time           | 4      | Unsigned Int (Epoch Time)      |
+| 5. Bits           | 4      | Internal Bitcoin Target Format |
+| 6. Nonce          | 4      | (Arbitrary Data)               |
+
+1. The *[block version][]{:#term-block-version}{:.term}* number indicates which set of block validation rules
+   to follow so Bitcoin Core developers can add features or
+   fix bugs. As of block height 227,836, all blocks use version number
+   2.
+
+2. The *hash of the previous block header* puts this block on the
+   block chain and ensures no previous block can be changed without also
+   changing this block's header.
+
+3. The *Merkle root* is a hash derived from hashes of all the
+   transactions included in this block. It ensures no transactions can
+   be modified in this block without changing the block header hash.
+
+4. The *[block time][]{:#term-block-time}{:.term}* is the approximate time when this block was created in
+   Unix Epoch time format (number of seconds elapsed since
+   1970-01-01T00:00 UTC). The time value must be greater than the
+   median time of the previous 11 blocks. No peer will accept a block with a
+   time currently more than two hours in the future according to the
+   peer's clock.
+
+5. *Bits* translates into the target threshold value---the maximum allowed
+   value for this block's hash. The bits value must match the network
+   difficulty at the time the block was mined.
+
+6. The *[header nonce][]{:#term-header-nonce}{:.term}* is an arbitrary input that miners can change to test different
+   hash values for the header until they find a hash value less than or
+   equal to the target threshold. If all values within the nonce's four
+   bytes are tested, the time can be updated or the
+   coinbase transaction can be changed and the Merkle
+   root updated.
+
+{% endautocrossref %}