Remove objects from cached association array at time of nested_attributes call instead of waiting until save

We add the objects to the cached association array at time of
call if we are creating them, so for consistency, we should delete
them from the cached association array at time of call if we are
deleting/removing them.  Issue pointed out by DaVinci on the Google
Group.
Fix issues in RDoc example code

This makes the example code valid ruby, so that it will be syntax
highlighted.

Some other minor RDoc issues fixed as well.