gh-38621: switch from `genbg` to `genbgL` in `nauty_genbg` to generate larger bipartite graphs.
Fixes #38618.
`genbg` was limited to bipartite graphs with up to 32 vertices. Using
`genbgL`, we can go to 64 vertices.
Note that in the doctests, we use the form `...genbg...` as currently
lines starting with `>A` contain `genbgL` while lines starting with `>E`
contain `genbg`. using this formulation, we are on the safe side.
On the way, we fix the documentation of `digraphs.tournaments_nauty()`.
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URL: https://github.com/sagemath/sage/pull/38621
Reported by: David Coudert
Reviewer(s): Matthias Köppe
