Fix cfgs in rustdoc

[joshlf]

FromZeros::new_vec_zeroed has the wrong cfg rendered on docs.rs:

We emit the wrong doc_cfg attribute:

zerocopy/src/lib.rs

Lines 1943 to 1946 in 2bec60b

	#[cfg(feature = "alloc")]
	#[cfg_attr(doc_cfg, doc(cfg(feature = "alloc")))]
	#[inline(always)]
	fn new_vec_zeroed(len: usize) -> Vec<Self>

We should fix this and also figure out whether there are other incorrect doc_cfg items to fix while we're here.

Mentoring instructions

Find and fix any instance of doc_cfg that doesn't match the corresponding cfg attribute.

[whee]

FWIW, I looked at the state of doc_cfg items in main at 5b99dc3 and did not see any mismatches anywhere.

The item mentioned here was fixed in e56a42b.

[joshlf]

Huh, seems you're right. Cool.