Document new default of `-Zrustdoc-scrape-examples`

[weihanglo]

Problem

#10343 introduces new behaviours for -Zrustdoc-scrape-examples, specifically, scraping examples by default under some circumstances. It would be helpful if the unstable doc can reflect this change a bit.

Proposed Solution

Update https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#scrape-examples

Notes

No response

[weihanglo]

cc @willcrichton, could you help this?

[willcrichton]

Yes I'll take care of it.