Edits to documentation

[kkotenko]

This

• changes some grammar (mostly adding articles)
• adds some formatting for inline code
• slight adjustments to wording

[codecov[bot]]

Codecov Report

Base: 95.43% // Head: 95.43% // No change to project coverage :thumbsup:

Coverage data is based on head (3c47eb9) compared to base (5eba5d6). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #587 +/- ## ======================================= Coverage 95.43% 95.43% ======================================= Files 49 49 Lines 1751 1751 Branches 189 189 ======================================= Hits 1671 1671 Misses 53 53 Partials 27 27 ``` 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=pytest-dev). 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=pytest-dev)

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