Add explanation of requirements for spline parameters in RQSpline docs

[Micki-D]

This adds an explanation of the requirements for the RQSpline parameters to ensure continuity and differentiability of the splines to the docstring of RQSpline.

For the sake of completeness, the previous description (about the spline being continued by a linear function with slope one) was incorrect, as the spline is continued by the identity function.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.02%. Comparing base (d1fe653) to head (bc234fd).
Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   97.02%   97.02%           
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  Files           6        6           
  Lines         404      404           
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  Hits          392      392           
  Misses         12       12           

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[oschulz]

Merci!