DigraphDegeneracy: improve doc

[mtorpey]

Some problems in the doc for DigraphDegeneracy and DigraphDegeneracyOrdering.

1. Grammar errors: "in later position of the list", "every induced of digraph", "of the vertices of the vertices of".
2. Why isn't digraph degeneracy just the maximum degree of the graph? These are equal in all provided examples, and I can't see what the difference is.
3. What's a "list ordering"? Do we define this somewhere else?

[james-d-mitchell]

2. Why isn't digraph degeneracy just the maximum cardinality of the graph? These are equal in all provided examples, and I can't see what the difference is.

I think there must be a word missing from the first sentence here, o/w what's the maximum cardinality of a graph?

[mtorpey]

Edited my opening comment.

[mtorpey]

I've looked this up on Wikipedia and I'm starting to believe that it isn't just maximum degree. A counterexample should be

Digraph([
[2, 3],
[1, 3, 9],
[1, 2],
[5, 6, 10, 13],
[4, 6, 7, 10, 14],
[4, 5, 7, 8, 9, 11],
[5, 6, 8],
[6, 7],
[2, 6],
[4, 5],
[6, 12, 15],
[11],
[4],
[5],
[11, 16],
[15]
])

which I believe has maximum degree 6 but degeneracy 2. Should maybe add this (or something) to the examples/tests.

[mtorpey]

1. I've fixed this in DigraphDegeneracy improvements #921.
2. I've satisfied myself of this now. It's just a case of carefully reading the definition.
3. Here ordering was the name of a variable. Probably fine, but anyway it's improved in DigraphDegeneracy improvements #921.