Improve JsDoc

[tarunbatra]

Improve JsDoc to support suggestions in VScode.

Inspiration: #10 and #12.

[katanacrimson]

Current JSDoc is also incorrect and seems to omit has() and not() argument details.

Also, not() seems to have an optional argument that is not marked optional even in the readme, but appears optional based on the provided example.

[tarunbatra]

@damianb

1. You are right; the params are missing. Thanks for the comment, it will help tracking the issue.
2. Ah, the param is marked optional using the square brackets around them, but not explicitly by mentioning optional as in the documentation of has. I think it's better to drop the explicit mention to be consistent.

[thakursaurabh1998]

@tarunbatra Adding the label good-first-issue will lead to listing of your repo in this page: https://github.com/topics/good-first-issue and someone interested for the good first issues might come and help :D

[tarunbatra]

Makes sense @thakursaurabh1998. Thanks for the suggestion.