feat: 'explain' shows Continuations and Placeholders for filters

[claremacrae]

Description

The explain instruction now successfully explains Continuations and Placeholders for filter lines.

Motivation and Context

See #2349 - for which this is a partial solution.

Still to do: explanations of Continuations and Placeholders for sort by function and group by function.

How has this been tested?

• New tests
• Lots of exploratory testing

Screenshots (if appropriate)

Changes in the documentation:

Types of changes

Changes visible to users:

• New feature (prefix: feat - non-breaking change which adds functionality)
• Documentation (prefix: docs - improvements to any documentation content for users)

Internal changes:

• Refactor (prefix: refactor - non-breaking change which only improves the design or structure of existing code, and making no changes to its external behaviour)
• 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