Document field types specifics (settings, columns...)

Each field type should provide an exhaustive description of
- their columns and indexes (when #415044: Indexes for field storage. gets in)
- their additional behavior on hook_field_load() (additional 'columns', filtering of invalid references...)
- their field and instance settings

Each formatter and widget should provide an exhaustive description of
- their settings
- for widgets, their '#properties' (when #362058: Field Types: Make widget FAPI elements field agnostic. gets in)

Ideally, those informations should not be scattered in various PHPdocs, but gathered at the beginning of each field-type module. Not sure about the exact format and structure, proposals welcome :-)