Update docs on zone configs to reflect updated multi-region capabilities

[rmloveland]

We need to update the docs for zone configs for 21.1 to reflect the changes that will happen as part of the improved high-level multi-region support, including (but not limited to):

• Update reference docs on meaning of new zone config setting keys

• Determine what are the complex interactions / validations that will will need to happen for each of the new keys listed below (as well as how that affects behavior of existing config fields), and document said interactions

• Related to the above: describe what happens when users modify the zone configs manually (this is discussed in the MR spec)

New keys:

• survive_failure
• num_voters config: introduce num_voters and voter_constraints cockroach#57184
• voting_constraints config: introduce num_voters and voter_constraints cockroach#57184
• non_blocking_transactions (rfc: Non-Blocking Transactions cockroach#52745)

Removed keys:

• +ssd since it will be overwritten by the new multiregion SQL commands (apparently - see MR Functional Spec)

[rmloveland]

This is complete, modulo an exact description of the multi-region validations. Since we are pointing most users to use the SQL API for their multi-region needs, punting on the validation explainer for now