Rename hash field to _hash and document it properly

doc/lua_api.txt

@@ -2040,6 +2040,12 @@ Some of the values in the key-value store are handled specially:
 * `palette_index`: If the item has a palette, this is used to get the
   current color from the palette.
 
+Item metadata is usually not fully sent to client (except the client has
+local map saving enabled or it is disabled globaly by the server). Only
+the fields `description`, `short_description`, `color`, `palette_index`
+and `tool_capabilities` are always sent. If there are any other fields,
+they will be hashed into an additional `_hash` field.
+
 Example:
 
     local meta = stack:get_meta()

src/itemstackmetadata.cpp

@@ -78,7 +78,7 @@ void ItemStackMetadata::serialize(std::ostream &os, InventoryOptimizationOption
 	}
 	std::string hash_str = os_hash.str();
 	if (! hash_str.empty()) {
-		os2 << "hash" << DESERIALIZE_KV_DELIM
+		os2 << "_hash" << DESERIALIZE_KV_DELIM
 			<< murmur_hash_64_ua(hash_str.data(), hash_str.length(), 0xdeadbeef) << DESERIALIZE_PAIR_DELIM;
 	}
 	os << serializeJsonStringIfNeeded(os2.str());