Fix raster docs based on Python docstrings

[mihaigurau-tomtom]

Just a few small edits of the docs for Raster Functions, particularly regarding descriptions of returns for rst_rastertogridcount and rst_rastertogridmin.

I think it'd be nice to update the images for the rst_rastertogrid* functions as well, as they seem to be inconsistent & now all link to the output of rst_rastertogridavg. However, I couldn't generate the images myself, hence that change may be part of a different PR.

Cheers! :)

[milos-colic]

@mihaigurau-tomtom We have now opened up the process for merging the external PRs.
Can you please confirm with the: https://github.com/databrickslabs/mosaic/blob/main/CONTRIBUTING.md?
After your PR is compliant, I am happy to review it and merge it if the review is successful.

[codecov]

Codecov Report

Merging #377 (1ef9e02) into main (bb49756) will not change coverage.
The diff coverage is n/a.

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  Coverage   94.99%   94.99%           
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  Files         198      198           
  Lines        5655     5655           
  Branches      181      181           
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  Hits         5372     5372           
  Misses        283      283           

[edurdevic]

Hi @mihaigurau-tomtom , in order for us to proceed it is sufficient to add a comment to this PR with your sign-off (Eg. Signed-off-by: Joe Smith <[email protected]>).
We need this to make sure you are aware of the current license, and that you agree with open sourcing it. :)

[CLAassistant]

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[CLAassistant]

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