Several Corrections & Clarifications Suggested On IRC

**Suggested by @cbeams:**

_includes/ref_block_chain.md:

    * Mention that coinbase is the first transaction in a block.

**Suggested by @gmaxwell:**

_includes/ref_core_rpcs-abcdefg.md:

    * Mention that you need to unlock your wallet when you run out of
      keys in the keypool.

    * Remove erroneous assertion that txindex=1 would allow
      `getreceivedbyaddress` to check balances of addresses not
      belonging to this wallet.

_includes/ref_transactions.md:

    * Clarify that OP_RETURN scripts aren't usually executed because
      they always return false.

en/developer-reference.md

    * Add a warning about using block chain or mempool data in
      executable context.
Mention that coinbase is the first transaction in a block in "Block Chain" subsection