Tag docs with feature flag required to use them

[joshka]

See https://doc.rust-lang.org/rustdoc/unstable-features.html\#doc_auto_cfg-automatically-generate-doccfg

[joshka]

Example:

And:

[joshka]

To test this, install:

• rust nightly
• cargo-docsrs (optional-ish, but useful)
• run cargo +nightly docsrs --open

[genusistimelord]

thank you, I just have not gotten to this yet so this is helpful. just recently made these changes to axum_session just have not gotten to my other libs yet.