doc: update FAQ for the arrival of parametric polymorphism

[robpike]

Many sections of the FAQ need tweaking, a few need replacement. Update before release.

For example, here's a question there that needs to be rethought: "Why does Go not have generic types?"

[gopherbot]

Change https://golang.org/cl/374215 mentions this issue: _content/doc/faq: add a couple of generics FAQs

[dmitshur]

CL 374215 resolves this issue, but isn't to be submitted until Go 1.18 is released.

Since there's nothing more to do here during 1.18 cycle, moving to 1.19 and adding early-in-cycle. Actually, we also have issue #49938 tracking this task, so we can can close this one.