Manual improvements

[tyhlee]

Mention independent/predictor variables and dependent/response variables at the same time at the top of the documentation.

Minor edits that do not change the context.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: +0.57 :tada:

Comparison is base (b728917) 88.75% compared to head (0e77089) 89.32%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #520 +/- ## ========================================== + Coverage 88.75% 89.32% +0.57% ========================================== Files 8 8 Lines 1040 1040 ========================================== + Hits 923 929 +6 + Misses 117 111 -6 ``` [see 1 file with indirect coverage changes](https://codecov.io/gh/JuliaStats/GLM.jl/pull/520/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaStats) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaStats). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaStats)

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[bkamins]

Thank you. Let us wait for GLM.jl maintainers to comment.

[ayushpatnaikgit]

@mousum-github