Validate Auto field's increment option

[subhashb]

Validate and document Auto field's increment functionality.

It seems to be applicable only to those database adapters that support it, and it needs be tightly coordinated with the values for IDENTITY_STRATEGY and IDENTITY_TYPE configuration values.

[subhashb]

Documentation has been updated in #437 to reflect aspects around increment functionality. For now, the decision is to retain it as part of the Auto field for those rare cases where we need an incrementing value column. This need not be a primary key.

The database schema has to be configured separately to increment the values of the attribute/column.