Minor English fixes

[mark-summerfield]

Using foo/bar/baz examples is fine for abstract thinkers but unhelpful for applied thinkers. Using "realistic" (but neutral, non-distracting) names works for applied thinkers and is just as easy for abstract thinkers, so in my view is to be preferred. For example, instead of struct Foo bar baz end, I'd have done, e.g., struct Tag name value end, etc. Similarly, I'd replace obj with child, parent, sibling or link. I haven't done this of course because this is a doc policy decision and I'm just doing English fixes. (Well I changed xx to data since the former looks like a 'fixme' tag.)

Another global thing I've noticed is that the docs clearly have multiple authors so there is a fair amount of stylistic inconsistency. Doesn't matter for online docs, but wouldn't be nice if turned into book form.

The .md files tend to have quite long line lengths. I have tried to preserve the lines to make the diffs simpler to read but personally I'd have used a maximum line width since that's easier to edit and diff across a wide range of editors.

The OrderedPair example shows how to enforce an invariant by throwing an exception. This is fine of course, but I think an additional or alternative example should be given that shows how to preserve an invariant where possible, e.g.,
OrderedPair(x,y) = x > y ? new(y,x) : new(x,y), or a different example altogether. Unless 'fixing' like this is bad practice in Julia?

Liked the OurRational and SummedArray examples and the explanations that followed them.

[KristofferC]

Thanks for working on this @mark-summerfield!

[ViralBShah]

This appears good to merge.

[mark-summerfield]

Thanks @KristofferC I really appreciate you saying so.