Publish docs for optional features on docs.rs

[georgeclaghorn]

The docs for the visitor module currently don't appear on docs.rs. Fix by enabling all optional features when generating docs for docs.rs. Annotate public items that require optional features.

To test locally, run:

RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --all-features --open

(Apologies if these docs are intentionally not published.)

References: Stack Overflow.