doc: streamline Matrix argument variants

[fingolfin]

This should make it a bit clearer when a matrix object is used for data, and when as an "example" or "reference" object. And by merging some entries, I also think it is a bit easier to understand the relations.

However, to really make this good, examples for all or at leas most of the variants ought to be added...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.66%. Comparing base (fa26016) to head (57474df).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6361      +/-   ##
==========================================
+ Coverage   78.63%   78.66%   +0.02%     
==========================================
  Files         684      684              
  Lines      292693   292720      +27     
  Branches     8660     8660              
==========================================
+ Hits       230159   230264     +105     
+ Misses      60726    60642      -84     
- Partials     1808     1814       +6     

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[ThomasBreuer]

The new description is better than the old one.
A few cross-references point to Matrix with label "for a list and a matrix object", this label does no longer exist.