Bug 1510602 - update build documentation to mention artifact builds r=firefox-build-system-reviewers,gps

Differential Revision: https://phabricator.services.mozilla.com/D13435

--HG--
extra : moz-landing-system : lando

python/mozboot/mozboot/bootstrap.py

@@ -40,7 +40,7 @@ APPLICATION_CHOICE = '''
 Note on Artifact Mode:
 
 Artifact builds download prebuilt C++ components rather than building
-them locally.
+them locally. Artifact builds are faster!
 
 Artifact builds are recommended for people working on Firefox or
 Firefox for Android frontends. They are unsuitable for those working