Correct calculation of `target_diameter_arcsec` for targets located very close to the observer

[ortk95]

Previous implementation produced produced incorrect/invalid values when target and observer were very close. The updated equation should now produce the correct value for any target/observer distance. Also updated documentation to explicitly state how this value is calculated (i.e. using arcsin(body.r_eq / body.target_distance)).

Pull request checklist

• [x] Add a clear description of the change
• [x] Add any new tests needed
• [x] Run spell check on new text visible to user (documentation, GUI etc.)
• [x] Check any changes to requirements.txt are reflected in setup.py and conda-forge feedstock
• [x] Check code passes CI checks (run run_ci.sh or check GitHub Actions)

See CONTRIBUTING.md for more details.

[coveralls]

Pull Request Test Coverage Report for Build 8265549026

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.573%

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Totals
Change from base Build 8265480670:	0.0%
Covered Lines:	2880
Relevant Lines:	2892

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💛 - Coveralls