add a bit to part 3 intro

Author: mark-i-m

src/part-3-intro.md

@@ -2,7 +2,9 @@
 
 This part describes the process of taking raw source code from the user and
 transforming it into various forms that the compiler can work with easily.
-These are called intermediate representations.
+These are called _intermediate representations (IRs)_.
 
 This process starts with compiler understanding what the user has asked for:
 parsing the command line arguments given and determining what it is to compile.
+After that, the compiler transforms the user input into a series of IRs that
+look progressively less like what the user wrote.