Docblock improvements and more in CRM_Core_Config and other classes.

My IDE (PhpStorm) sees a lot of yellow and quite some red, when looking at CiviCRM code.
Time to do something about it!
This is going to be a mammoth task, but someone has to start.

I am mostly following the Drupal coding standards here, as far as they deviate from PSR standards.
E.g. I put param descriptions on a new line, like this:

```php
  /**
   * @param string $userFramework
   *   One of 'Drupal', 'Joomla', etc.
   */
```

One obvious goal is to allow as much compile-time type checking as possible, which can help identify many bugs already during development.

I am also removing some redundant docblock stuff like "@public". Noone needs that, the method signature is good enough.

The PR is split into multiple commits, which hopefully makes it easier to review, or pick only parts of it.