Document ions at zero-area nodes.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (9df433b) 88.38% compared to head (cf01a1a) 88.38%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1164 +/- ## ======================================= Coverage 88.38% 88.38% ======================================= Files 176 176 Lines 12946 12946 ======================================= Hits 11442 11442 Misses 1504 1504 ```

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[bbpbuildbot]

Logfiles from GitLab pipeline #195302 (:white_check_mark:) have been uploaded here!

Status and direct links:

• :white_check_mark: spack_setup
• :white_check_mark: build:intel
• :white_check_mark: build:nvhpc
• :white_check_mark: test:intel
• :white_check_mark: test:nvhpc