rpc: render Type::ANY in help text instead of aborting

RPCResult::Type::ANY triggers NONFATAL_UNREACHABLE() in ToSections(),
which crashes the help() RPC when a command uses Type::ANY in a
nested result field.

Previously this was never hit because Type::ANY was only used as a
top-level alternate result type, filtered out before ToSections() is
called.

getopenrpcinfo() will use this result type, so render it like other
types allowing it to be used in nested result definitions like schema.

src/rpc/util.cpp

@@ -1032,7 +1032,8 @@ void RPCResult::ToSections(Sections& sections, const OuterType outer_type, const
 
     switch (m_type) {
     case Type::ANY: {
-        NONFATAL_UNREACHABLE(); // Only for testing
+        sections.PushSection({indent + maybe_key + "xxx" + maybe_separator, Description("any")});
+        return;
     }
     case Type::NONE: {
         sections.PushSection({indent + "null" + maybe_separator, Description("json null")});