Merge #3690

3690: ra_hir: add more docs r=matklad a=Veetaha

Cited from [hear](https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Fwg-rls-2.2E0/topic/hover/near/191446937)

Co-authored-by: veetaha <veetaha2@gmail.com>

Author: bors[bot]

crates/ra_hir/src/lib.rs

@@ -4,6 +4,18 @@
 //! The principal difference between HIR and syntax trees is that HIR is bound
 //! to a particular crate instance. That is, it has cfg flags and features
 //! applied. So, the relation between syntax and HIR is many-to-one.
+//!
+//! HIR is the public API of the all of the compiler logic above syntax trees.
+//! It is written in "OO" style. Each type is self contained (as in, it knows it's
+//! parents and full context). It should be "clean code".
+//!
+//! `ra_hir_*` crates are the implementation of the compiler logic.
+//! They are written in "ECS" style, with relatively little abstractions.
+//! Many types are not self-contained, and explicitly use local indexes, arenas, etc.
+//!
+//! `ra_hir` is what insulates the "we don't know how to actually write an incremental compiler"
+//! from the ide with completions, hovers, etc. It is a (soft, internal) boundary:
+//! https://www.tedinski.com/2018/02/06/system-boundaries.html.
 
 #![recursion_limit = "512"]