Tooltip text for Fields in schema definition.

## The problem to solve
Apostrophe needs a simple tooltip system for displaying hints when hovering field name or label
Is your feature request related to a problem you are experiencing that is *not* a bug? Please describe.
No way to express a tooltip found. Maybe exists.
> A clear and concise description of what the problem is. Ex: "When I do [some task], I find it frustrating when [something happens]." If this is a bug report (things are actually breaking), open a bug report instead.

## Proposed solution
```
// FIELDS
....
myfield: {
label: 'my field label',
...
tooltip: 'Extensive text or method'
}
....
```
Describe the solution you'd like

> A clear and concise description of what you want to happen.

## Alternatives

Describe alternatives you've considered

> A clear and concise description of any alternative solutions or features you've considered. For example, should this be done on the project level instead?

## Additional context

> Mention any other context or attach screenshots regarding the feature request.
> Check the existing issue labels and add any you think are relevant, e.g.,
> `UI`, `UX`, `accessibility`, `security`.

No files or tests are named. Start by locating how schema field definitions are rendered and how field labels are displayed, then trace the existing UI behavior for hover hints. Done means a field definition can provide tooltip text, including the proposed string or method form, and users see it when hovering the field name or label.

Written by the indexing model from the issue text.