Reviewed and edited part3 narrative
Reviewed and edited part3 narrative

This patch reviews the part3 narrative. In particular:

- Use `-` for bullet points (not `*`).
- Use `zend-*` notation when refering to ZF components.
- Remove class-level docblocks if they do not provide details.
- Use `{@inheritDoc}` in implementation docblocks.
- Use `int` and `bool` for typehints.
- Added a note about local configuration files when discussing database credentials.
- Detail the "why" behind the storage interface.
- Provide background information on the TDG pattern.
- Provide some background on the result set prototype and zend-hydrator.
- Provide background information on the Repository pattern and why it's used.
- Rephrasing and rewording for flow.
- Minor code edits for CS and internal consistency with project code.

lowtower/zend-expressive-tutorial,lowtower/zend-expressive-tutorial