Add docstring for MeshRegion class.

[lorenzovecchietti]

Close #3721.

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[codecov]

Codecov Report

Merging #3785 (504ffb3) into main (44d83df) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #3785   +/-   ##
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  Coverage   81.07%   81.07%           
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  Files         180      180           
  Lines       61977    61977           
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+ Hits        50249    50250    +1     
+ Misses      11728    11727    -1     

[lorenzovecchietti]

As commented here:

I added a docstring for the MeshRegion class.
So when a user creates the mesh region, he can see the different attributes and their meaning with the help command. I also explicitly wrote down available options for particular arguments, like the one related to this issue.

The alternative was to create setter and getter, but this seems harder to maintain with little benefit.