Note removal of validation message translation.
Add examples of using __() to translate validation messages.
Tidy up the existing docs for bake.

* Remove 1.3 'new' features.
* Remove incorrect information.
First pass at updating docs for adding bake tasks.
Fix wording.
Remove sr from PDF builds.

The default PDF packages don't understand sr as a locale.
Merge pull request #1251 from cakephp/3.0-bake

3.0 Update docs for extract task and bake.