minor #6360 Editing the Doctrine section to improve accuracy and readability (natechicago)

This PR was merged into the 2.3 branch.

Discussion
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Editing the Doctrine section to improve accuracy and readability

By clarifying sections of the documentation that are a little ambiguous or poorly-worded, these changes should make Symfony's introduction to Doctrine more accurate, expressive, and easier to read, particularly for users who have never worked with Doctrine before.

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | n/a

Commits
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8bfb9e0 Editing the Doctrine section to improve readability.
b80cf6d Editing the Doctrine section to improve readability.
2464a67 Editing the Doctrine section to improve readability.

Author: weaverryan