Better docstrings for probs and counts

[kahaaga]

Fixes the remainder of #370.

• There was no mention of Counts at all in the Probabilities doctoring, so the probs_or_counts argument in the docstring wasn't really making much sense. Therefore, I split the constructor signature up into two lines, and explicitly reference Counts.
• Clarified that when inputting Counts without specifying outcomes, then the outcomes are inherited from Counts.
• Slight rewording and a few extra words to make it explicit what "probabilities correspond to outcomes that describe axes" mean.
• Added a few short examples of constructing a Probabilities (show, don't tell). This point can be omitted, but I find it useful.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (80202b9) 88.68% compared to head (62da385) 88.68%.

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Additional details and impacted files

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##             main     #382   +/-   ##
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  Coverage   88.68%   88.68%           
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  Files          80       80           
  Lines        2359     2359           
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  Hits         2092     2092           
  Misses        267      267           

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