Docs: explicitly state that `_sharing` is not inherited and defaults to `private`

[mitselek]

The docs describe _sharing defaults and _inheritrights propagation in two separate sections of src/overview/entities/index.md, but neither section explicitly states the combined behaviour:

An entity with no explicit _sharing value defaults to private. _sharing is not propagated via _inheritrights — only _owner, _editor, _expander, and _viewer are inherited.

The pieces are inferrable:

• "Sharing" section: private is listed as "the default"
• "Rights Inheritance" section: only the four access-rights properties are listed as inherited; _sharing is not mentioned

But without an explicit statement, integrators naturally assume _sharing inherits from the parent (like the four rights do when _inheritrights: true). We've now seen two independent teams hit this — one assumed deleting a child's _sharing would make it "inherit private from the parent," and designed a bulk-restrict operation around that model. The operation happened to work (absent = default private), but the mental model was wrong, and it would have led to incorrect predictions about future entity creation under parents with non-private _sharing.

Suggested addition

In the "Sharing" section, after the _sharing value table and the existing ::: info block, add:

::: info
`_sharing` is per-entity only — it is **not** propagated to children via `_inheritrights`. An entity with no explicit `_sharing` value defaults to `private`, regardless of its parent's `_sharing`.
:::

This is a one-line clarification that prevents the most common misunderstanding. Happy to send a docs PR if preferred.