docs-bug(mat-tab-nav-panel): Documentation implies that [tabPanel] is optional, but must be specified.

### Documentation Feedback

The documentation implies that [tabPanel] can be left unprovided by saying "If not provided, then the nav bar follows the ARIA link / navigation landmark pattern".

However, if left unprovided, it will throw an error stating: "A mat-tab-nav-panel must be specified via [tabPanel]".

### Affected documentation page

https://material.angular.io/components/tabs/api#MatTabNav

Start with the MatTabNav section on the affected API documentation page and compare its wording about [tabPanel] with the reported error. Update the documentation so it accurately describes whether [tabPanel] is required, then verify the page no longer implies that it can be omitted.

Written by the indexing model from the issue text.