docs: Add docs for task properties isBlocked() and isBlocking()

[claremacrae]

Description

Add tests and docs for the following new task properties, for use in custom filters, sorts and groups:

• task.isBlocked(query.allTasks)
• task.isBlocking(query.allTasks)

Both need the argument query.allTasks.

Motivation and Context

Getting dependencies ready to release

How has this been tested?

• New tests
• Exploratory testing

Screenshots (if appropriate)

Types of changes

Changes visible to users:

• Documentation (prefix: docs - improvements to any documentation content for users)

Internal changes:

• Tests (prefix: test - additions and improvements to unit tests and the smoke tests)

Checklist

• My code follows the code style of this project and passes yarn run lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• My change has adequate Unit Test coverage.

Terms

• My contribution follow this project's contributing guide
• I agree to follow this project's Code of Conduct