Improved documentation for PAT

[lipkau]

Description

Documentation on the usage of PAT for authenticating with Jira Cloud APIs is not clear about what value to use as value for user in the credentials.

Motivation and Context

closes #353

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [ ] My code follows the code style of this project.
• [ ] I have added Pester Tests that describe what my changes should do.
• [x] I have updated the documentation accordingly.