expand compilation stages

src/how-to-build-and-run.md

@@ -2,7 +2,7 @@
 
 The compiler is built using a tool called `x.py`. You will need to
 have Python installed to run it. But before we get to that, if you're going to
-be hacking on rustc, you'll want to tweak the configuration of the compiler.
+be hacking on `rustc`, you'll want to tweak the configuration of the compiler.
 The default configuration is oriented towards running the compiler as a user,
 not a developer.
 
@@ -55,13 +55,20 @@ compiler to compile the newer version. In particular, the newer version of the
 compiler, `libstd`, and other tooling may use some unstable features
 internally. The result is the compiling `rustc` is done in stages.
 
-- **Stage 0:** the stage0 compiler is the current _beta_ compiler; we
-  download this binary from the internet.
-- **Stage 1:** the code in your clone is then compiled with the stage
-  0 compiler to produce the stage 1 compiler.
-- **Stage 2:** the code in your clone is then compiled with the stage
-  1 compiler *again* to produce the stage 2 compiler (i.e. it builds
-  itself).
+- **Stage 0:** the stage0 compiler can be your existing (perhaps older version of)
+  Rust compiler, the current _beta_ compiler or you may download the binary
+  from the internet
+- **Stage 1:** the code in your clone (for new version)
+  is then compiled with the stage0
+  compiler to produce the stage1 compiler.
+  However, it was built with an older compiler (stage0),
+  so to optimize the stage1 compiler we go to next stage
+- **Stage 2:** we rebuild our stage1 compiler with itself
+  to produce the stage2 compiler (i.e. it builds
+  itself) to have all the _latest optimizations_
+- _(Optional)_ **Stage 3**: to sanity check of our new compiler, we can build it again
+  with stage2 compiler which must be identical to itself,
+  unless something has broken
 
 For hacking, often building the stage 1 compiler is enough, but for
 final testing and release, the stage 2 compiler is used.