DOC: Clarify satpy.yaml use with nested dictionaries

[gerritholl]

In the satpy config documentation, clarify how dotted keys such as readers.clip_negative_radiances should be configured in satpy.yaml.

• Fully documented

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.10%. Comparing base (38d106d) to head (5d44f27).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3036   +/-   ##
=======================================
  Coverage   96.10%   96.10%           
=======================================
  Files         381      381           
  Lines       55544    55544           
=======================================
  Hits        53383    53383           
  Misses       2161     2161           

Flag	Coverage Δ
behaviourtests	3.89% <ø> (ø)
unittests	96.20% <ø> (ø)

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[coveralls]

Pull Request Test Coverage Report for Build 12884265898

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.214%

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Totals
Change from base Build 12870674719:	0.0%
Covered Lines:	53629
Relevant Lines:	55739

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💛 - Coveralls