rpc: update validateaddress RPCExamples to bech32

also contains the following changes:
- rpc: factor out example bech32 address for RPCExamples
- doc: update developer notes wrt RPCExamples addresses
 (mention the EXAMPLE_ADDRESS constant as an example for an invalid bech32
  address suitable for RPCExamples help documentation)

src/rpc/misc.cpp

@@ -42,8 +42,8 @@ static UniValue validateaddress(const JSONRPCRequest& request)
             "}\n"
                 },
                 RPCExamples{
-                    HelpExampleCli("validateaddress", "\"1PSSGeFHDnKNxiEyFrD1wcEaHr9hrQDDWc\"")
-            + HelpExampleRpc("validateaddress", "\"1PSSGeFHDnKNxiEyFrD1wcEaHr9hrQDDWc\"")
+                    HelpExampleCli("validateaddress", EXAMPLE_ADDRESS) +
+                    HelpExampleRpc("validateaddress", EXAMPLE_ADDRESS)
                 },
             }.Check(request);
 

src/rpc/util.cpp

@@ -14,6 +14,7 @@
 #include <tuple>
 
 const std::string UNIX_EPOCH_TIME = "UNIX epoch time";
+const std::string EXAMPLE_ADDRESS = "\"bc1q09vm5lfy0j5reeulh4x5752q25uqqvz34hufdl\"";
 
 void RPCTypeCheck(const UniValue& params,
                   const std::list<UniValueType>& typesExpected,

src/rpc/util.h

@@ -28,6 +28,12 @@
  */
 extern const std::string UNIX_EPOCH_TIME;
 
+/**
+ * Example bech32 address used in multiple RPCExamples. The address is intentionally
+ * invalid to prevent accidental transactions by users.
+ */
+extern const std::string EXAMPLE_ADDRESS;
+
 class FillableSigningProvider;
 class CPubKey;
 class CScript;