Clarify documentation of z_getbalanceforviewingkey for Sprout viewing keys.

doc/release-notes.md

@@ -48,8 +48,10 @@ New RPC Methods
   process table; `zcashd-wallet-tool` is specifically provided to avoid this
   problem.
 - 'z_getbalanceforviewingkey' This newly created API allows a user to obtain
-  balance information for funds visible to a Sprout, Sapling, or Unified full
-  viewing key.
+  balance information for funds visible to a Sapling or Unified full
+  viewing key; if a Sprout viewing key is provided, this method allows 
+  retrieval of the balance only in the case that the wallet controls the
+  corresponding spending key.
 
 RPC Changes
 -----------

src/wallet/rpcwallet.cpp

@@ -3627,6 +3627,7 @@ UniValue z_getbalanceforviewingkey(const UniValue& params, bool fHelp)
         throw runtime_error(
             "z_getbalanceforviewingkey \"fvk\" ( minconf )\n"
             "\nReturns the per-pool balances viewable by a full viewing key known to the node's wallet."
+            "\nSprout viewing keys may be used only if the wallet controls the corresponding spending key."
             "\nArguments:\n"
             "1. \"fvk\"        (string) The selected full viewing key.\n"
             "2. minconf      (numeric, optional, default=1) Only include transactions confirmed at least this many times.\n"