docs: add dev vs prod details

Fix #5315

Author: filipesilva

docs/documentation/build.md

@@ -59,10 +59,34 @@ ng build --base-href /myUrl/
 ng build --bh /myUrl/
 ```
 
-### Bundling
-
-All builds make use of bundling, and using the `--prod` flag in  `ng build --prod`
-or `ng serve --prod` will also make use of uglifying and tree-shaking functionality.
+### Bundling & Tree-Shaking
+
+All builds make use of bundling and limited tree-shaking, while `--prod` builds also run limited
+dead code elimination via UglifyJS.
+
+### `--dev` vs `--prod` builds
+
+Both `--dev`/`--target=development` and `--prod`/`--target=production` are 'meta' flags, that set other flags.
+If you do not specify either you will get the `--dev` defaults.
+
+`--dev`:
+- `--aot` is set to `false`.
+- `--environment` is set to `dev`.
+- `--output-hashing` is set to `media`.
+- `--sourcemaps` is set to `true`.
+- `--extract-css` is set to `false`.
+
+`--prod`:
+- `--aot` is set to `true`.
+- `--environment` is set to `prod`.
+- `--output-hashing` is set to `all`.
+- `--sourcemaps` is set to `false`.
+- `--extract-css` is set to `true`.
+
+`--prod` also sets these non-flaggable settings:
+- Adds service worker if configured in `.angular-cli.json`.
+- Replaces `process.env.NODE_ENV` in modules with the `production` value (this is needed for some libraries, like react).
+- Runs UglifyJS on the code.
 
 ## Options
 <details>