Rollup merge of rust-lang#148115 - fmease:rustdoc-no-capture, r=notriddle

rustdoc: Rename unstable option `--nocapture` to `--no-capture` in accordance with `libtest`

Context: rust-lang#133073, rust-lang#139224 (TL;DR: `libtest` has soft-deprecated `--nocapture` in favor a new & stable `--no-capture`; we should follow suit).

Since the rustdoc flag is unstable (tracking issue: rust-lang#148116), we're allowed to remove the old flag immediately. However since the flag has existed for 4 years we could hard-deprecate the flag first or at least be considerate and provide a diagnostic referring users to the new flag. This PR does neither. Let me know what you would think would be best.

Cargo doesn't use this flag, not yet at least (rust-lang/cargo#9705), so we really are free to sunset this flag without bigger consequences.

Author: jhpratt