rpc, doc: update listdescriptors RCP help

This RPC lists all the descriptors present in the wallet, not only
the ones that were imported, but also the ones generated when a
new wallet is created.

It can be verified by creating a new wallet and calling the
`listdescriptors` RPC, which will contain 8 ranged descriptors that
are created for every new wallet.

Also, update the description to get rid of "descriptor-enabled"
because this is the only wallet type available now after removal of
legacy wallets.

src/wallet/rpc/backup.cpp

@@ -462,7 +462,7 @@ RPCHelpMan listdescriptors()
 {
     return RPCHelpMan{
         "listdescriptors",
-        "List descriptors imported into a descriptor-enabled wallet.\n",
+        "List all descriptors present in a wallet.\n",
         {
             {"private", RPCArg::Type::BOOL, RPCArg::Default{false}, "Show private descriptors."}
         },