Add RPC documentation for getblockheader[chainwork].

Author: jamesob

src/rpcblockchain.cpp

@@ -323,7 +323,8 @@ UniValue getblockheader(const UniValue& params, bool fHelp)
             "  \"bits\" : \"1d00ffff\", (string) The bits\n"
             "  \"difficulty\" : x.xxx,  (numeric) The difficulty\n"
             "  \"previousblockhash\" : \"hash\",  (string) The hash of the previous block\n"
-            "  \"nextblockhash\" : \"hash\"       (string) The hash of the next block\n"
+            "  \"nextblockhash\" : \"hash\",      (string) The hash of the next block\n"
+            "  \"chainwork\" : \"0000...1f3\"     (string) Expected number of hashes required to produce the current chain (in hex)\n"
             "}\n"
             "\nResult (for verbose=false):\n"
             "\"data\"             (string) A string that is serialized, hex-encoded data for block 'hash'.\n"