First version of a doc about API scope.

[alainjobart]

@sougou first draft, let's talk about it, or you can comment.

[sougou]

Looks like it covers everything we discussed. The scope of a shard query is still being debated. I can add the details once that's finalized.

[michael-berlin]

The 0 shard is a special case which will always create confusion. Hence I suggest to better call it out e.g.

For unsharded keyspaces, or custom sharded keyspaces, the shard names have traditionally been numbers e.g. 0. (It is important to note that Vitess does not interpret these shards (including shard name 0) as range-based shard.)