[2pt] feedback: API reference | no "unavailable" field, but there's "unknown"

[TarantoolBot]

Product: Tarantool, vshard
Since: TBD
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_rock/vshard/vshard_api/#router-public-api
SME: @ Gerold103

Details

<…>s known to the router and available for read and write requests
|unavailable – the number of buckets known to the router but unavailable for any requests
unreachable – the number of buckets whose replica sets are not known to the router|

Example:
COPYvshard.router.info()tarantool> vshard.router.info<…>

There's no "unavailable" field, but there's "unknown". Also, the explanation is obscure. I guess unreachable and unknown descriptions are mixed up.

@rosik

Related to #2259