do not assume readers know about JSR-380 in TS Binder docs

[vlukashov]

The TypeScript form binder docs should not assume reader are familiar with the JSR-380 (aka Bean Validation 2.0). They should include a one-line explanation of what JSR-380 (and make the link to the spec more visible), and also briefly describe supported annotations.

DoD:

• the title of the Using JSR 380 Bean Validation docs page is updated so that ppl not familiar with JSR-380 also understand what this article is about
• the link to the JSR-380 spec is higher in the page and is more visible
• the example also includes usage of @Size