Fix star general
Description
The commit corrects some small problems with the star general stencil: - The document was not correct, as thenumber of extra points was not matching what the code does - The number of coefficient in the polynomial was too big and hence useless (193 instead of 109).
News snippet
Minor bug and documentation fixes.
Checklist
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I have checked that my code follows the Octopus coding standards - [] I have added tests for all the new features added in this request.