Clean up doc blocks.

Remove some words that I don't think need to be there.
Add assert to clarify what happened.

I was a tiny bit confused as to what the expected outcome was. This
additional assertion made it clearer to me.
Handle composite primary keys when merging many.

mergeMany() needs to be composite key aware as tables/associations can
easily have composite keys. I've used `;` as a separator as I've
not seen many composite keys with them, and even when they do an out of
place `;` will still enforce uniqueness.

Refs #4817
Merge pull request #4875 from cakephp/issue-4817

Fix mergeMany() and composite keys.