Update js-api.md (#827)
Update js-api.md (#827)

* Update js-api.md

Looking at the source https://github.com/cssinjs/jss/blob/dd20297caba69ac4c390f1460dca0034691f4a2a/packages/jss/src/Jss.js#L47 options are merged on setup not completely overwritten.

I'm also asking because `react-jss` expects the options to be public readonly and the typings over at DefinitelyTyped do not have options listed either. 

All of which is no direct concern of you but I'd like to get some feedback before I work on PRs on those packages.

* Update js-api.md

Improve wording. Shallow merge means shallowMerge(a, b) = ({ ...a, ...b})

* Update js-api.md

Duplicate is not very thorough anyway.

cssinjs/jss,jsstyles/jss,cssinjs/jss,jsstyles/jss