Fix documentation and unit conversion for Geosphere 10minute radiation data

[gutzbenj]

Dear @mhuber89,

this patch should accommodate the needs to fix the documentation as well as unit conversion mentioned in #988 .

To ensure the right conversion factor I looked it up here: https://hextobinary.com/unit/heatflux/from/wpm2/to/jsm2

Please check the changes and comment below!

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.05% ⚠️

Comparison is base (2b97ab4) 90.92% compared to head (329853f) 90.88%.

❗ Current head 329853f differs from pull request most recent head fd2c97d. Consider uploading reports for the commit fd2c97d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #998      +/-   ##
==========================================
- Coverage   90.92%   90.88%   -0.05%     
==========================================
  Files         105      105              
  Lines        8716     8718       +2     
  Branches      997      998       +1     
==========================================
- Hits         7925     7923       -2     
- Misses        591      595       +4     
  Partials      200      200              

Flag	Coverage Δ
unittests	90.88% <100.00%> (-0.05%)	⬇️

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

Files Changed	Coverage Δ
wetterdienst/provider/geosphere/observation/api.py	99.48% <100.00%> (+<0.01%)	⬆️

... and 1 file with indirect coverage changes

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