Fixes #2254:

1. Fixes original issue: AJAX and client validation doesn't work with `errorSummary` set to true.
2. Added `CForm::$showErrors` which controls whether error elements of the each attribute should be rendered.
PHPDoc fixes:

1. Additional `CForm` class note.
2. Better description of the `CForm::$showErrors`.
Additional explanation of what `CForm::$showErrors` precisely does.
PHPDoc of `CForm::$showErrors` is now a bit less cryptic.
Merge pull request #2320 from resurtm/fixes-2254

Fixes #2254: AJAX and client validation doesn't work with `errorSummary` set to true.