Update ref. manual to favor constructor injection

The breakout box named "Constructor-based or setter-based DI?" in the
reference manual currently recommends the use of setter injection.

This commit refines this text to align with current best practices and
now favors constructor injection over setter injection.

Issue: SPR-11459
Merge pull request #466 from olivergierke/SPR-11459

* SPR-11459:
  Update ref. manual to favor constructor injection