docs: document Enterprise ACL inspection functions

[glasstiger]

Summary

Document the Enterprise ACL inspection functions introduced by questdb/questdb-enterprise#1222 and make the Access control functions page a complete reference for all four ACL table functions:

Function	What it returns
all_permissions()	Available permission names and grantable levels
permissions()	Effective permissions for the current or a named entity
active_permissions()	Effective permissions across persisted users, groups, and service accounts, including group inheritance
active_grants()	Direct grants across those entities, without inherited permissions

The reference now includes an Enterprise marker, formal function grammar, result columns, authorization requirements, limitations, and precise database-, table-, and column-scope semantics. Verified SQL examples and result tables cover:

• comparing effective permissions with direct grants;
• finding entities that can read a table or a specific column;
• preserving group rows when auditing SSO/OIDC access;
• finding direct grant recipients and entities with the grant option.

Limitations explain access paths not represented directly, including views and assumed service accounts.

Related changes:

• Consolidate the three SHOW PERMISSIONS headings into one section covering current and named entities, add a result example, and preserve the old #show-permissions-for-current-user and #show-permissions-user deep links.
• Add inbound links from RBAC, SHOW, GRANT, and REVOKE, plus stable active_permissions and active_grants compatibility anchors.
• Correct the Enterprise quick start: all_permissions() lists available permissions rather than applied grants.
• Clarify when LIST USERS, USER DETAILS, or both are required.
• Add the Access control functions page to the Functions sidebar and documentation changelog.

Dependencies

• The new functions depend on questdb/questdb-enterprise#1222.
• Web Console autocomplete and syntax highlighting for active_permissions() and active_grants() require questdb/sql-parser#37.

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