docs(pivot): mark gridSize override internal

[damyanpetev]

Currently visible in https://www.infragistics.com/products/ignite-ui-angular/docs/typescript/latest/classes/IgxPivotGridComponent.html when it shouldn't be.

Additional information (check all that apply):

• [ ] Bug fix
• [ ] New functionality
• [x] Documentation
• [ ] Demos
• [ ] CI/CD

Checklist:

• [x] All relevant tags have been applied to this PR
• [ ] This PR includes unit tests covering all the new code (test guidelines)
• [ ] This PR includes API docs for newly added methods/properties (api docs guidelines)
• [ ] This PR includes feature/README.MD updates for the feature docs
• [ ] This PR includes general feature table updates in the root README.MD
• [ ] This PR includes CHANGELOG.MD updates for newly added functionality
• [ ] This PR contains breaking changes
• [ ] This PR includes ng update migrations for the breaking changes (migrations guidelines)
• [ ] This PR includes behavioral changes and the feature specification has been updated with them