path, doc: Explain "expected properties"

[Trott]

In the path.format() documentation, it refers to the "expected properties" of pathObject. Add a sentence or so explaining what those "expected properties" are or link to another part of the doc that explains such things.

Alternatively, change that sentence to specify the dir and base properties rather than "expected properties" since those are actually the only two that matter in the context of the sentence that says "expected properties".

This is a good first contribution for someone interested in getting familiar with the contribution process.

[eversojk]

I can take a look at this as well along with #5747.

[Trott]

We have a PR for this so I'll remove good first contribution so no one else tries to do it too!