Mention Bitcoin's RPC Hash Byte Order

* Add a short subsection about the different byte orders used with
  hashes in Bitcoin Core and other software.

* Re-word some text in other sections to mention the byte order
  differences

This commit created based on comments from @SergioDemianLerner (thanks!)

_includes/ref_core_rpc_intro.md

@@ -0,0 +1,92 @@
+### Hash Byte Order
+
+{% autocrossref %}
+
+Bitcoin Core RPCs accept and return hashes in the reverse of their
+normal byte order. For example, the Unix `sha256sum` command would display the
+SHA256(SHA256()) hash of Mainnet block 300,000's header as the
+following string:
+
+    5472ac8b1187bfcf91d6d218bbda1eb2405d7c55f1f8cc820000000000000000
+
+The string above is also how the hash appears in the
+previous-header-hash part of block 300,001's header:
+
+<pre>02000000<b>5472ac8b1187bfcf91d6d218bbda1eb2405d7c55f1f8cc82000\
+0000000000000</b>ab0aaa377ca3f49b1545e2ae6b0667a08f42e72d8c24ae\
+237140e28f14f3bb7c6bcc6d536c890019edd83ccf</pre>
+
+However Bitcoin RPCs use the reverse byte order for hashes, so if you
+want to get information about block 300,000 using the `getblock` RPC,
+you need to reverse the byte order:
+
+    > bitcoin-cli getblock \
+      000000000000000082ccf8f1557c5d40b21edabb18d2d691cfbf87118bac7254
+
+(Note: hex representation uses two characters to display each byte of
+data, which is why the reversed string looks somewhat mangled.)
+
+The rational for the reversal is unknown, but it likely stems from
+Bitcoin's use of hash digests (which are byte arrays in C++) as integers
+for the purpose of determining whether the hash is below the network
+target. Whatever the reason for reversing header hashes, the reversal
+also extends to other hashes used in RPCs, such as TXIDs and merkle
+roots. 
+
+Off-site documentation such as the Bitcoin Wiki tends to use the terms
+big endian and little endian as shown in the table below, but they
+aren't always consistent. Worse, these two different ways of
+representing a hash digest can confuse anyone who looks at the Bitcoin
+Core source code and finds a so-called "big endian" value being stored
+in a little-endian data type.
+
+As header hashes and TXIDs are widely used as global identifiers in
+other Bitcoin software, this reversal of hashes has become the standard
+way to refer to certain objects. The table below should make clear where
+each byte order is used.
+
+<div style="text-align: left;" markdown="1">
+
+|---------------+---------------------|-----------------|
+| Data | Internal Byte Order ("Big Endian") | RPC Byte Order ("Little Endian") |
+|---------------|---------------------|-----------------|
+| Example: SHA256(SHA256(0x00))  | Hash: 1406...539a         | Hash: 9a53...0614     |
+|---------------|---------------------|-----------------|
+| Header Hashes: SHA256(SHA256(block header))  | Used when constructing block headers  | Used by RPCs such as `getblock`; widely used in block explorers |
+|---------------|---------------------|-----------------|
+| merkle Roots: SHA256(SHA256(TXIDs and merkle rows))  | Used when constructing block headers  | Returned by RPCs such as `getblock` |
+|---------------|---------------------|-----------------|
+| TXIDs: SHA256(SHA256(transaction))  | Used in transaction inputs | Used by RPCs such as `gettransaction` and transaction data parts of `getblock`; widely used in wallet programs |
+|---------------|---------------------|-----------------|
+| P2PKH Hashes: RIPEMD160(SHA256(pubkey))  | Used in both addresses and pubkey scripts  | **N/A:** RPCs use addresses which use internal byte order |
+|---------------|---------------------|-----------------|
+| P2SH Hashes: RIPEMD160(SHA256(redeem script))  | Used in both addresses and pubkey scripts | **N/A:** RPCs use addresses which use internal byte order |
+|---------------|---------------------|-----------------|
+
+</div>
+
+Note: RPCs which return raw results, such as `getrawtransaction` or the
+raw mode of `getblock`, always display hashes as they appear in blocks
+(internal byte order).
+
+The code below may help you check byte order by generating hashes
+from raw hex.
+{% endautocrossref %}
+
+{% highlight python %}
+#!/usr/bin/env python
+
+from sys import byteorder
+from hashlib import sha256
+
+## You can put in $data an 80-byte block header to get its header hash,
+## or a raw transaction to get its txid
+data = "00".decode("hex")
+hash = sha256(sha256(data).digest()).digest()
+
+print "Warning: this code only tested on a little-endian x86_64 arch"
+print
+print "System byte order:", byteorder
+print "Internal-Byte-Order Hash: ", hash.encode('hex_codec')
+print "RPC-Byte-Order Hash:      ", hash[::-1].encode('hex_codec')
+{% endhighlight %}