Docs: clarify format of varint used in ocdbt

The specification for the OCDBT format https://google.github.io/tensorstore/kvstore/ocdbt/ makes several references to varint without stating what looks like in memory. Is it a [VLQ](https://en.wikipedia.org/wiki/Variable-length_quantity), a [LEB128](https://en.wikipedia.org/wiki/LEB128) (signed or unsigned?), or anything else?