Augment sort.jl documentation with doctests

[xorJane]

I added some doctests to sort.jl in issorted(), sort!(), sort(), sortrows(), sortcols(), sortperm(), and sortperm!(). Per make -C doc doctest, it seems all added tests pass.

Many of the examples are redundant. Just let me know if you'd like more/different examples, less redundancy, etc.!

Thanks a lot!!

ref #18821

[mauro3]

I think they should be more concise: when calling help at the REPL I don't want pages and pages of examples. For instance there is no advantage to sort vectors with more than three elements. Also construction of the vector should be done inside the function call. For example the issorted example could be shortened to:

```jldoctest
julia> issorted([1,2,3])
true

julia> issorted(-1* [1,2,3])
false

Thanks for your efforts!

[StefanKarpinski]

It's much better to have too much documentation than too little, but I do think that some of the extra examples would be better off in the manual, but then again the stdlib reference is generated from the doc strings. Maybe we need some mechanism for basic and extended documentation?

[xorJane]

Thank you --- I appreciate the feedback!

I updated my PR to make the examples more concise as @mauro3 suggested, and I added doctests for sortperm() and sortperm!()

For now I left vector construction outside of the function calls to sort!() and sortperm!() to illustrate mutation, but I'm happy to change that too. :)

[hayd]

Another thing worth mentioning is that [-1, -2, -3] is "sorted" in the sense that it is reversed, so it may be worth adding even more examples:

julia> issorted([1, 2, 3])
true

julia> issorted([2, 1, 3])
false

julia> issorted([-1, -2, -3])
false

julia> issorted([-1, -2, -3], rev=true)
true

This branch needs a rebase!

[xorJane]

Thanks for the bump! Just rebased. It looks like most of the functions for which I originally wrote doctests have since been covered. Now this PR just adds doctests for sortrows and sortcols, and adds a doctest using the rev flag to the docstring for issorted as suggested by hayd. Have a great night!

[tkelman]

lgtm, thanks for updating

[coveralls]

Coverage remained the same at ?% when pulling 78cc622 on xorJane:sortdocs into 55f4417 on JuliaLang:master.