doc: Add comment to use ValueFromAmount/AmountFromValue for JSON, not utilmoneystr

src/utilmoneystr.h

@@ -14,6 +14,9 @@
 
 #include "amount.h"
 
+/* Do not use these functions to represent or parse monetary amounts to or from
+ * JSON but use AmountFromValue and ValueFromAmount for that.
+ */
 std::string FormatMoney(const CAmount& n);
 bool ParseMoney(const std::string& str, CAmount& nRet);
 bool ParseMoney(const char* pszIn, CAmount& nRet);