Rollup merge of #68161 - ruuda:fix-instant-docs, r=rkruppe

Fix system call docs for time::Instant

The link for UNIX was pointing to the Cloud ABI docs. It should have been pointing to the `clock_gettime` docs instead. A similar table is repeated in the docs for `SystemTime`, but there the UNIX entry was already correct.

`clock_gettime(CLOCK_MONOTONIC)` is the current implementation: https://github.com/rust-lang/rust/blob/3ebcfa1451cfedc13a07e6353d8ade9742dfdc2a/src/libstd/sys/unix/time.rs#L274
https://github.com/rust-lang/rust/blob/3ebcfa1451cfedc13a07e6353d8ade9742dfdc2a/src/libstd/sys/unix/time.rs#L348-L352

r? @steveklabnik