Feature flags docs

[molpopgen]

We have the docs building for all the public API.
This means that it is not immediately obvious that some
API bits are due to feature flags.

We should have a section in the doc of each item:

# Feature flags

This thing requires the cargo feature "foo"

[juntyr]

Since docs are built using nightly on docs.rs, you could make use of the following:

https://doc.rust-lang.org/beta/unstable-book/language-features/doc-cfg.html

https://docs.rs/about/builds

i.e. enable a feature on docs builds, that enabled the unstable doc_cfg feature, which allows you to add #[doc(cfg(...))] to any public item which is cfg-ed.

See https://docs.rs/syn/latest/syn/ for an example of what that looks like

[molpopgen]

Thanks! I'd just read this in "Rust for Rustaceans", which is what got me thinking about it.

[molpopgen]

It does seem like there's some "trust" involved that this will work. I'm unable to get the docs.rs commands from their README to work locally to confirm that the changes actually do anything.