Cleanup collections::slice documentation.

This does a few things:

 - remove references to ~[] and the OwnedVector trait, which are both
   obsolete
 - correct the docs to say that this is the slice module, not the vec
   module
 - add a sentence pointing out that vectors are distinct from Vec
 - remove documentation on Vec.

closes #15459
This example may be a bit confusing because these are slices, not vectors.