- moved the logic within _BindParamClause which decides about coercing the bind's...

based on the compared type into AbstractType.   The new method is called _coerce_compared_value()
and receives the operator and the raw python value to be coerced.    TypeDecorator
overrides this to provude the "old" 0.5 behavior of coercing the other side of the expression
unconditonally.
- added docs to TypeDecorator attempting to explain this though they are a little verbose.
- added caveats to Interval, which in "non-native" mode can't really handle proper expression behavior.  the "typing" would have to move into the compiler, such that on a DB like SQLite we're actually wrapping left/right side into its epoch functions, etc., its a big job.  current infrastructure, as powerful as it is, still isn't at that level.