Add section on span status for databases

[lmolkova]

Addresses the DB part of #1536.

Let's polish it for databases and then expand to other conventions (and potentially have a generic guidance).

The intention is to keep exception event part experimental since

• there doesn't seem to be a consensus or a common approach for it
• span events is going away

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

testing codeowners