Documentation: what exactly is meant by "irregular grid"?

Maybe this is just a problem of wording

The Readme and the [documentation](https://juliamath.github.io/Interpolations.jl/latest/#Irregular-Grids) tells me that "Currently this package supports [...] irregular grids."
I interpreted this (wrongly as it turns out) that I can interpolate points arbitrarily scattered. It seems I'm not alone with this: this issue #552 clarifies that "irregular" means "rectilinear", and not arbitrary irregular.

Writing explicitly:
"Currently this package supports interpolation on Cartesian and rectilinear grids"
would have helped a lot.

For reference:
[Wikipedia for Regular grids](https://en.wikipedia.org/wiki/Regular_grid#Rectilinear_grid)
[Wikipedia for Unstructured grid or irregular grid](https://en.wikipedia.org/wiki/Unstructured_grid)