EAMxx: in infra, ensure comments/code reflect generality of masking treatment

comments like

https://github.com/E3SM-Project/E3SM/blob/1ad1b5f473a728b45ce85a5ad1d0776000ae0b04/components/eamxx/src/share/grid/remap/coarsening_remapper.cpp#L273-L274

may make it sound like this is the only case supported. I suggest a quick walk through the infra code (remap, io, etc.) to reflect the new reality of more extensive masking we are now using.

Make the assumption clearer and more general, e.g.,

```cpp
// NOTE that in eamxx, masking is at most rank-2, either on COL only or on COLxLEV only
```

Even better, add documentation to clarify how masking works and use cases under components/eamxx/docs.

--

@bartgol I didn't assign anyone here, because this is the type of thing a curious reader may be able to do and document ... or you know, maybe that one helpful 🤖🤖🤖🤖🤖🤖🤖🤖🤖 ;) (copilot, if you're reading...)

ran into this in #7652 with annoying layout-like errors that took a bit of time to trace...

Start with components/eamxx/src/share/grid/remap/coarsening_remapper.cpp around lines 273-274, then walk through the remap and io infrastructure mentioned in the issue. Review components/eamxx/docs for a suitable place to document masking behavior and use cases. Done means comments and documentation clearly describe the supported general masking treatment.

Written by the indexing model from the issue text.