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 ;)

该 issue 没有指定文件或测试。找到解释 fieldwire 和 size 的教程部分，然后记录字段编号大于 15 时会使用额外的 varint 字节，并以 writingprogram (16) 和 source (17) 为例。完成标准是读者能够清楚理解这个额外字节以及解码行为。