Explicit namespaces in sort-related docstrings

[jishnub]

This represents the unexported names by the fully qualified namespace, which makes it simpler to interpret the keyword arguments. Otherwise, for example, it's not obvious where Forward comes from in the sort docstring.

Also, replaces some occurrences of instance <: Algorithm by instance isa Algorithm, which is the correct relation. This is also closer to English, which helps with readability.

Additionally, I think Base.Sort.defalg should be documented, as this is being used in the function signatures. This may be done separately.

[LilithHafner]

These seem like improvements

Changing CountingSort <: Algorithm to CountingSort() isa Algorithm would not be an improvement on its own, as the declaration of Counting sort is literally struct CountingSort <: Algorithm end, but I guess it makes sense for consistency.

This also documents the internals from a Base user's perspective which, absent the public keyword, would be a problem, but with that it's clear these are not public so it's okay to document them in a way that focuses on the experience of Base users poking around.

Overall, LGTM. Thanks!

[jishnub]

Personally I don't prefer the CountingSort() isa Algorithm over the current style either, but I did indeed change this for consistency. I suppose this form makes it clear that the constructor doesn't take any argument.