[DOC] Update dgl_graph.cc

[hadevin]

Update _contrib_csr_neighbor_non_uniform_sample and contrib_csr_neighbor_uniform_sample documentation to indicate that the expected number of arrays for seed_arrays is the num_args minus two (csr matrix and probability).

Description

Update the documentation per issue #18989

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[mxnet-bot]

Hey @hadevin , Thanks for submitting the PR
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[szha]

Thanks for the contribution!