docs(libflux): document fluxcore::semantic::bootsrap module

[pierwill]

Removes #![allow(missing_docs)].

The meaning of this module was not entirely clear to me. Can we say more about what "bootstrap" means here? 🤔

[pierwill]

Example: The flux crate calls bootstrap::infer_stdlib(). Does this make sense with my description of the module as "Flux start-up"?

[jsternberg]

One caveat:

Example: The flux crate calls bootstrap::infer_stdlib(). Does this make sense with my description of the module as "Flux start-up"?

Bootstrap happens at compile time and doesn't happen at runtime. The bootstrap infers the types and is used to embed the results into the resulting library so they can be accessed more quickly.