added documentation for PyDP naive bayes comparison

[dnabanita7]

Description

added docs for PyDP comparison

Checklist

• [x] I have followed the Contribution Guidelines and Code of Conduct
• [x] I have commented my code following the OpenMined Styleguide
• [ ] I have labeled this PR with the relevant Type labels
• [x] My changes are covered by tests

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[grilhami]

@dnabanita7 Great work on adding those descriptions! However, I would like to suggest some minor adjustments.

1. I think it would be a good idea to mention the description of some variables in a code block like this. For example, instead of just "clf", we can do clf. I think that would help make the description more readable.

2. For the part where mean_train_accs, mean_test_accs, and dp_results, it'll be great if we can specify that those results are differentially private since the GaussianNB class from PyDP is differentially private.

[dnabanita7]

cc @chinmayshah99 @grilhami