Remove incoherence between Doctrine and Propel introduction paragraphs

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4349

The introductory paragraph of Doctrine and Propel sections, although both are true in some way, look contradictory, so a rephrase of at least one of the two paragraphs would be convenient.