Document fully_consistent nuances on CockroachDB

[ivanauth]

Fixes #525

Adds callouts to the consistency and CockroachDB datastore pages
explaining that fully_consistent does not guarantee read-after-write
on CockroachDB due to cluster_logical_timestamp() clock skew across
nodes.

[miparnisari]

line 103 ends with "If you need read-after-write consistency, consider using a [ZedToken]"

can we merge this callout with that one?

[miparnisari]

Suggested change

	SpiceDB selects a revision using CockroachDB's `cluster_logical_timestamp()` at request time.
	Because CockroachDB is a distributed database, this timestamp can differ from other nodes by up to the cluster's configured [`max_offset`][max-offset] (default: 500ms).
	If the node handling the read picks a timestamp older than the one assigned to a recent write on a different node, the read will not see that write even though the data has been committed.
	At request time, SpiceDB directs the query towards one node in the CockroachDB cluster, and then runs [`cluster_logical_timestamp()`](https://www.cockroachlabs.com/docs/stable/functions-and-operators#system-info-functions).
	However, because CockroachDB is a distributed database, this timestamp can differ from other nodes by up to the cluster's configured [`max_offset`][max-offset] (default: 500ms).
	If the node handling the read picks a timestamp older than the one assigned to a recent write on a different node, the read will not see that write even though the data has been committed.