Add :since option to schema

[github-actions[bot]]

Pull Request Test Coverage Report for Build 83af397924cb271cea9bcbcabed72c8940df666c-PR-119

• 5 of 6 (83.33%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 92.739%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/nimble_options/docs.ex	5	6	83.33%
<!--	Total:	5	6	83.33%	-->

Totals
Change from base Build 46e70d7e890bb40faa7be672430c052c19046c0f:	-0.2%
Covered Lines:	281
Relevant Lines:	303

---

💛 - Coveralls

[josevalim]

It can be added to the docs in a written format, no? So what is the benefit? :)

[whatyouhide]

Mostly to push people to use it, but that's a good point. Let's drop this?

[josevalim]

Yup, dropping. :)