collections: update docs of slice get() and friends
collections: update docs of slice get() and friends

for the new SliceIndex trait.  Also made the docs of the unchecked
versions a bit clearer; they return a reference, not an "unsafe
pointer".

GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust