This commit introduces a function `uniqueind` that is a slight modification of the existing `unique` function accepting a `dim` argument but returns an additional three index vectors.

The outputs, as described in the docstring, are as follows:

`C` - an array of the unique elements/rows/columns/hyperplanes of the input AbstractArray `itr` along the dimension `dim`
`ia` - a vector of index values such that slice indexing into `itr`along dimension `dim` re-produces `C`.  
        For example, if `itr` is a vector, `itr[ia] == C` returns `true`
`ib` - a vector of vectors of integers, where each vector of integers contains the index positions of the unique elements/rows/columns/planes within `C`
`ic` - a vector of index values such that slice indexing into `C` along dimension `dim` re-rproduces `itr`
        For example, if `itr` is a vector, `C[ic] == itr` returns `true`

The presence of `ia`, `ib`, and `ic` are meant to mimic the behaviors (but with potentially different output order) of various unique and group functions found in a few other technical computing languages.  This function provides all three of those outputs from a single function, while some other languages might only produce one or two arrays of index vectors from a given function.

It might be decided upon review that instead of a single function returning all four outputs, there should be multiple functions that return separate output arguments and more closely mimic similar functions from other languages.

This PR is in relation to issue #1845.

Removing a trailing whitespace that caused a build failure.

Missed a few last time.

One more trailing whitespace.

This commit makes the simple doc updates suggested in #14142 to change `itr` to `A` and use a fixed set of integers, instead of random integers, when constructing the multidimensional array examples.

Updating function name to `uniqueslices` primarily just to kick off a new round of Travis and AppVeyor tests, because the Travis tests failed last time for reasons seemingly unrelated to the contents of this file.