Docstrings for flux_godunov and flux_ec

[DanielDoehring]

Since #2204 there is now flux_ec mentioned in a docstring:

Trixi.jl/src/equations/numerical_fluxes.jl

Line 228 in f09a707

must be used together with an entropy-conservative two-point flux function (e.g., `flux_ec`) to yield

although such a flux is not explicitly documented. For completeness, I added the corresponding docstring for the Burgers equation.

In the same go, docstrings for flux_godunov are also added.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.37%. Comparing base (f09a707) to head (9e68b05).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2219   +/-   ##
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  Coverage   96.37%   96.37%           
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  Files         486      486           
  Lines       39186    39186           
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  Hits        37764    37764           
  Misses       1422     1422           

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unittests	96.37% <ø> (ø)

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