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.

## Also Discussed

Not changed in this pull, but also discussed in IRC:

* Add "beta" disclaimer to the docs. @saivann has already shown me a preview; we're just refining it a bit.

* Proper use of bitcoin values in programs (avoid floats) added to the TODO list: https://github.com/bitcoin/bitcoin.org/wiki/Documentation-TODO