Document flavors, feature-flags, and default exports

[filipdulic]

[filipdulic]

• flavors - flavors.md - flavors/mod.rs
• arc_swap - arc_swap.md - flavors/arc_swap.rs
• rwlock - rwlock.md - flavors/rwlock.rs
• atomic_arc - atomic_arc.md - flavors/atomic_arc.md

user cargo-modoc and modoc.config to generate doc-strings from markdown files in appropriate source files.

[filipdulic]

Ignore the abovo, doc-strings in rs files directly are probably a better solution