**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.
Thanks also (in alphabetical order) to @cbeams, @mikehearn, and
@tgeller, among others.
The last pre-squash commit was: c2b8d562aa107c7b68c60946cea14cdccc5159ad
