Better code docs for DataDictionary classes

[dafeder]

We have a whole API now for altering DB tables in the Drupal\datastore\DataDictionary namespace but very sparse documentation on how to use it. For instance as pointed out by @paul-m we have no guidance for how to use the arguments like $fields in AlterTableQueryBase.

Lets do a pass at these classes related to altering existing tables and add better explanations and examples to the class and method doc blocks.