Clarify `bootstrap_expect` default value

### Nomad version
All - Docs issue

### Operating system and Environment details
N/A

### Issue
The Nomad docs list `bootstrap_expect` as a required config value in the server stanza (https://www.nomadproject.io/docs/configuration/server#bootstrap_expect) with no default; when in fact this value is not required, and the current default value if unset is `0`.

If the intended behavior is to force the operator to set this explicitly, then Nomad should fail to start with an error message if it is not provided. If 0 is intended as a reasonable default, this should be documented.

### Reproduction steps
Start a nomad server instance with `bootstrap_expect` omitted. Nomad will start and behave as if the value is `0`.

#### Expected Result
Nomad fails to start and logs an error.

#### Actual Result
Nomad starts and and either does not bootstrap (versions below 0.11.0 or 1.0.2 and above) or bootstraps as a standalone instance (versions 0.11.0-1.0.2).

Open the server stanza documentation at the linked bootstrap_expect section and verify how an omitted value behaves across the Nomad versions described. Update the documentation so the required status and default are accurate, and ensure the reproduction behavior and version distinctions are reflected when the result is clear.

Written by the indexing model from the issue text.