Doc string terminology for unnamed distributions created via .dist()

When updating the docstrings for pm.AR() distribution class (https://github.com/pymc-devs/pymc/pull/6080), it became clear that we have no clean, unified way to refer to the "unnamed distribution" arguments taken by some distributions (e.g., time series, pm.Mixture, etc.). @ricardoV94 suggested adopting something like the "tensor_like" nomenclature used elsewhere. This issue is intended to start discussion about whether that's a good idea and, if so, what a good term might be. While we're at it, we might try and figure out if there are any other argument types that we might want to name.