aes: `--cfg aes_backend` documentation is no longer accurate

[TechnoPorg]

aes/lib.rs still mentions setting the aes_backend cfg to avx256 or avx512, but these options were removed in #580.

block-ciphers/aes/src/lib.rs

Lines 48 to 49 in 2449027

	//! Note: Enabling VAES256 or VAES512 still requires specifying `--cfg
	//! aes_backend = "avx256"` or `--cfg aes_backend = "avx512"` explicitly.

I'd happily open a PR to fix this, but one thing I'm not sure about is what to do about the use of this flag in CI, where you have some tests that were previously explicitly forcing the VAES backends, and now auto-detect them like anywhere else:

block-ciphers/.github/workflows/aes.yml

Lines 105 to 112 in 2449027

	vaes256:
	runs-on: ubuntu-latest
	strategy:
	matrix:
	include:
	- target: x86_64-unknown-linux-gnu
	rust: stable
	RUSTFLAGS: '-Dwarnings --cfg aes_backend="avx256"'