- if @classproperty is used with a regular class-bound

- if @classproperty is used with a regular class-bound
mapper property attribute, it will be called to get the
actual attribute value during initialization. Currently,
there's no advantage to using @classproperty on a column
or relationship attribute of a declarative class that
isn't a mixin - evaluation is at the same time as if
@classproperty weren't used. But here we at least allow
it to function as expected.
- docs for column_property() with declarative
- mixin docs in declarative made more clear - mixins
are optional - each subsection starts with, "in *declarative mixins*",
to reduce confusion