Correct Mutability of LineDigraph

[mtorpey]

In 2ffaaae (2019) @wilfwilson modified LineDigraph to output a digraph with the same mutability as the input. This PR updates the documentation to reflect this, and adds a test for it.

This brings constructors.gi code coverage to 100%.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.41%. Comparing base (5246e0c) to head (8d667a9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #926   +/-   ##
=======================================
  Coverage   97.40%   97.41%           
=======================================
  Files          50       50           
  Lines       21193    21193           
  Branches      639      639           
=======================================
+ Hits        20643    20645    +2     
+ Misses        485      483    -2     
  Partials       65       65           

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