Document feature flags for a given release

[pdaoust]

With unstable features moving behind feature flags that are off by default, the flags (and the means to enable them) should be documented. The means is already documented in Holonix; there's a DRY question there.

[ThetaSinner]

Example of documenting feature flags in docs is here https://crates.io/crates/wasmer

[pdaoust]

AC: In a discoverable location (my plan is to put it on an 0.4 upgrade guide, plus the top level module docs for relevant crates), all the feature flags are documented, along with instructions (or a link to the instructions on the Holonix readme) on how to use them.

[pdaoust]

Resolved in #499