improve documentation of tree_reduce

[rbruenig]

PR for improving documentation of tree_reduce, it will close #946

This PR only changes documentation and does not touch any actual code.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.64%. Comparing base (6814180) to head (44a73c5).
Report is 100 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@ ## master #955 +/- ## ========================================== + Coverage 94.38% 94.64% +0.26%  ========================================== Files 48 49 +1 Lines 6665 7065 +400 ========================================== + Hits 6291 6687 +396  - Misses 374 378 +4 

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[rbruenig]

@Philippe-Cholet Is there anything that still should be improved or is this ready to be merged 🙂 ?

[Philippe-Cholet]

I checked out your branch and opened the doc to read it. The text is fine, only got 2 visual nitpicks. I'll merge once fixed, thanks for the remainder.

[Philippe-Cholet]

Thanks for the needed clarification!