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.

_includes/ref_block_chain.md

@@ -27,7 +27,7 @@ As of version 2 blocks, each block consists of four root elements:
 
 4. One or more transactions.
 
-Every block must include a [coinbase transaction][]{:#term-coinbase-tx}{:.term} which should collect and
+The first transaction in a block must be 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

_includes/ref_core_rpcs-abcdefg.md

@@ -786,7 +786,9 @@ encryptwallet <passphrase>
 
 Encrypts the wallet with 'passphrase'. This is only to enable encryption
 for the first time.  After encryption is enabled, you will need to
-enter the passphrase to use private keys.
+enter the passphrase to use private keys (which includes generating
+additional new addresses once the keypool is exhausted---see
+`keypoolrefill`).
 
 *Warning:* there is no RPC to completely disable encryption.  If you
 want to return to an unencrypted wallet, you must create a new wallet
@@ -2657,8 +2659,7 @@ transactions with at least the indicated number of confirmations.
 {% autocrossref %}
 
 *String; required:* a Bitcoin address to check.  Must be an address 
-belonging to the wallet unless `txindex=1` was added to the `bitcoind`
-startup options.
+belonging to the wallet.
 
 {% endautocrossref %}
 

_includes/ref_transactions.md

@@ -47,9 +47,9 @@ The op codes used in standard transactions are,
     error in the Bitcoin Core implementation. This value is not used, so
     scriptSigs prefix the signatures with a single OP_0 (0x00).
 
-* [`OP_RETURN`][op_return]{:#term-op-return}{:.term} terminates the script in failure. However, this will not
-  invalidate a null data transaction which contains no more than 40
-  bytes following `OP_RETURN` no more than once per transaction.
+* [`OP_RETURN`][op_return]{:#term-op-return}{:.term} terminates the script in failure,
+  rendering the output unspendable and allowing a miner to claim the
+  satoshis sent to that OP_RETURN output as an additional transaction fee.
 
 A complete list of OP codes can be found on the Bitcoin Wiki [Script
 Page][wiki script], with an authoritative list in the `opcodetype` enum

en/developer-reference.md

@@ -33,6 +33,15 @@ title: "Developer Reference - Bitcoin"
 
 ### Remote Procedure Calls (RPCs)
 
+**Warning:** the block chain and memory pool can include arbitrary data
+which several of the commands below will return in hex format. If you
+convert this data to another format in an executable context, it could
+be used in an exploit. For example, displaying an output script as
+ASCII text in a webpage could add arbitrary Javascript to that page and
+create a cross-site scripting (XSS) exploit. To avoid problems, please
+treat block chain and memory pool data as an arbitrary input from an
+untrusted source.
+
 {% include ref_core_rpcs-abcdefg.md %}
 {% include ref_core_rpcs-hijklmn.md %}
 {% include ref_core_rpcs-opqrst.md %}