modal.md

## uni.showModal(options) @showmodal

<!-- UTSAPIJSON.modal.description -->

<!-- UTSAPIJSON.modal.param -->

<!-- UTSAPIJSON.modal.returnValue -->

<!-- UTSAPIJSON.modal.compatibility -->

<!-- UTSAPIJSON.modal.tutorial -->

<!-- UTSAPIJSON.general_type.name -->

<!-- UTSAPIJSON.general_type.param -->