Several Corrections & Clarifications Suggested On IRC

[harding]

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

[saivann]

@harding I have just found another occurence where it might be useful to clarify that coinbase tx is the first one in a block, feel free to import this commit: 6c9cf4f

Otherwise, LGTM! Thanks!

[harding]

@saivann done. Thanks!

[harding]

Note: this pull will be merged into #393 around 23:59 UTC tonight unless critical feedback is received.

[cbeams]

LGTM, thanks Dave.