reword HIR intro. Fix rust-lang#116

Author: mark-i-m

src/hir.md

@@ -1,11 +1,12 @@
 # The HIR
 
-The HIR – "High-level IR" – is the primary IR used in most of rustc.
-It is a desugared version of the "abstract syntax tree" (AST) that is generated
-after parsing, macro expansion, and name resolution have completed. Many parts
-of HIR resemble Rust surface syntax quite closely, with the exception that some
-of Rust's expression forms have been desugared away (as an example, `for` loops
-are converted into a `loop` and do not appear in the HIR).
+The HIR – "High-level IR" – is the primary IR used in most of rustc. It is a
+compiler-friendly representation of the abstract syntax tree (AST) that is
+generated after parsing, macro expansion, and name resolution have completed.
+Many parts of HIR resemble Rust surface syntax quite closely, with the
+exception that some of Rust's expression forms have been desugared away. For
+example, `for` loops are converted into a `loop` and do not appear in the HIR.
+This makes HIR more amenable to analysis than a normal AST.
 
 This chapter covers the main concepts of the HIR.