Update documentation for project versions in \Drupal\Core\Extension\InfoParserInterface::parse

Problem/Motivation

The feature added in #2205271: Project namespace for dependencies was missing some documentation of the new 'project' key. Let's add it here.

Steps to reproduce

Proposed resolution

In \Drupal\Core\Extension\InfoParserInterface::parse change
* - (versions): Version information, consisting of one or more
to
* - (versions): (optional) Version information, consisting of one or more

Remaining tasks

MR with the recommended change
Review
Commit

User interface changes

API changes

Data model changes

Release notes snippet