don't use non-ascii ' chars in documentation
deal with `#append` and `#prepend` on association collections.

Closes #7364.

Collection associations behave similar to Arrays. However there is no
way to prepend records. And to append one should use `<<`. Before this
patch `#append` and `#prepend` did not add the record to the loaded
association.

`#append` now behaves like `<<` and `#prepend` is not defined.
Merge pull request #9510 from senny/7364_warn_when_appending_prepending_to_an_association

deal with `#append` and `#prepend` on association collections