sql: Auto-generate docs for session variables

[jseldess]

We currently auto-generate doc files for cluster settings and functions and operators. It'd be wonderful to auto-generate session variable docs as well. Currently, we manually maintain docs on session variables that can be viewed and set, and, as a result, our docs are usually behind reality.

Jira issue: CRDB-4679

[github-actions]

We have marked this issue as stale because it has been inactive for
18 months. If this issue is still relevant, removing the stale label
or adding a comment will keep it active. Otherwise, we'll close it in
5 days to keep the issue queue tidy. Thank you for your contribution
to CockroachDB!

[knz]

still relevant