Update Scene.save_datasets to clarify what will be saved

[djhoese]

See #1138.

• Closes save_datasets does not save dependencies available due to unload=False #1138
• Fully documented

[codecov]

Codecov Report

Merging #1864 (da7d133) into main (ab7f3b1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1864   +/-   ##
=======================================
  Coverage   93.17%   93.17%           
=======================================
  Files         270      270           
  Lines       40315    40315           
=======================================
  Hits        37562    37562           
  Misses       2753     2753           

Flag	Coverage Δ
behaviourtests	4.65% <ø> (ø)
unittests	93.74% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/scene.py	92.32% <ø> (ø)

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[coveralls]

Coverage remained the same at 93.676% when pulling da7d133 on djhoese:doc-save-datasets-wishlist-only into ab7f3b1 on pytroll:main.

[gerritholl]

Looks good to me, thank you for the clarification.