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 may be a bit confusing in that this claims Vectors are the list type, but down below it's said that they're a distinct type. Could this wording be tightened up to stick with slices?