Add JSDoc to document attributes for camel case properties

[web-padawan]

Description

When using lit-analyzer for linting code that uses vaadin-combo-box, there are warnings for certain attributes.

Expected outcome

No warnings when using attributes corresponding to camel case properties.

Actual outcome

The following warnings are shown:

    Unknown attribute 'item-value-path'. Did you mean 'itemValuePath'?
    20:  item-value-path="lastName"
    no-unknown-attribute


    Unknown attribute 'item-label-path'. Did you mean 'itemLabelPath'?
    21:  item-label-path="lastName"

Steps to reproduce

1. Clone the repo: https://github.com/web-padawan/ts-vaadin-examples
2. Run yarn to install dependencies
3. Run `yarn lint:lit

Suggested fix

Add the following to JSDoc annotations for corresponding properties:

* @attr {string} item-label-path

* @attr {string} item-value-path