Update build system docs with the latest changes

Author: eregon

docs/build_system.md

@@ -16,9 +16,9 @@ The main solution for the second point seems a Makefile, otherwise many of the u
 ## General Design
 
 1. Templates are generated by `templates/template.rb`
-4. The `Makefile` compiles both `libprism.a` and `libprism.{so,dylib,dll}` from the `src/**/*.c` and `include/**/*.h` files
-5. The `Rakefile` `:compile` task ensures the above prerequisites are done, then calls `make`,
-  and uses `Rake::ExtensionTask` to compile the C extension (using its `extconf.rb`), which uses `libprism.a`
+2. The `Makefile` compiles both `libprism.a` and `libprism.{so,dylib,dll}` from the `src/**/*.c` and `include/**/*.h` files
+3. The `Rakefile` `:compile` task ensures the above prerequisites are done, then calls `make`,
+   and uses `Rake::ExtensionTask` to compile the C extension (using its `extconf.rb`)
 
 This way there is minimal duplication, and each layer builds on the previous one and has its own responsibilities.
 
@@ -35,14 +35,11 @@ loaded per process (i.e., at most one version of the prism *gem* loaded in a pro
 ### Building the prism gem by `gem install/bundle install`
 
 The gem contains the pre-generated templates.
-When installing the gem, `extconf.rb` is used and that:
-* runs `make build/libprism.a`
-* compiles the C extension with mkmf
-
-When installing the gem on JRuby and TruffleRuby, no C extension is built, so instead of the last step,
-there is Ruby code using FFI which uses `libprism.{so,dylib,dll}`
-to implement the same methods as the C extension, but using serialization instead of many native calls/accesses
-(JRuby does not support C extensions, serialization is faster on TruffleRuby than the C extension).
+
+When installing the gem on CRuby, `extconf.rb` is used and that compiles the C extension with mkmf, including both the extension files and the sources of prism itself.
+
+When installing the gem on JRuby and TruffleRuby, no C extension is built, so instead the `extconf.rb` runs `make build/libprism.{so,dylib,dll}`.
+There is Ruby code using FFI which uses `libprism.{so,dylib,dll}` to implement the same methods as the C extension, but using serialization instead of many native calls/accesses (JRuby does not support C extensions, serialization is faster on TruffleRuby than the C extension).
 
 ### Building the prism gem from git, e.g. `gem "prism", github: "ruby/prism"`