Fix in documentation

[bmitc]

The documentation mentioned join and not in, so the documentation has been corrected. join is the more common name here, so I'd be amenable to that change. But this PR just makes a start at addressing this by correcting the documentation to the currently defined name. It appears that in is intended to be used in a pipeline, as in <path> | in <directory>, which is why the name is slightly off for normal use of joining two paths.

[JakeSiFive]

The next wake is going to be a major version anyway so I think its fine to rename this to join but please run some large build to ensure people aren't using this for something else in a conflicting way.

[JakeSiFive]

Approving in case you don't want to rename it right now

[bmitc]

I'm okay with performing the rename later if there's agreement on actually doing the rename. Given other priorities, I'd probably prefer doing the rename later, just so that we can be careful since it's likely used in a lot of places, as you mentioned.

In other words, I'm good with just the documentation fix if you are. 🙂

[JakeSiFive]

Sounds good to me, go ahead and land it