Warn about the multiple bytes fields (writingprogram, source, osmosis_replication_*...)

Hello,

I think it could be good to warn when a field number exceeds 15, it needs more bytes to be encoded exactly like varints.

It happens for example on the writingprogram field ( = 16) which is encoded on two bytes 0x82 0x01.
MSB here is reserved to indicate the value also encoded on the next byte.
Decoding it like a varint is the way.

Also on source field ( = 17), and basically every field number greater than 15.

It personnaly took a while to understand why there was another byte between fieldwire and size and I think it'll be great to write it down to help a byte more ;)

この issue ではファイルもテストも指定されていません。fieldwire と size を説明しているチュートリアルのセクションを見つけ、フィールド番号が 15 を超える場合は追加の varint バイトを使用することを、writingprogram (16) と source (17) を例として文書化してください。追加のバイトとデコードの動作が読者に明確になれば完了です。