docs(@angular/cli): document output dir deletion

Address concerns in angular#4366, angular#6375.

Author: filipesilva

docs/documentation/build.md

@@ -13,6 +13,10 @@ ng build
 
 The build artifacts will be stored in the `dist/` directory.
 
+All commands that build or serve your project, `ng build/serve/e2e`, will delete the output
+directory (`dist/` by default).
+This can be disabled via the `--no-delete-output-path` (or `--delete-output-path=false`) flag.
+
 ### Build Targets and Environment Files
 
 `ng build` can specify both a build target (`--target=production` or `--target=development`) and an