Clarify image name nomenclature: what exactly is a 'tag'?

Given an image called foo:8080/bar:baz:

• Some parts of the Docker documentation use the term 'tag' to mean the whole thing, while others use it to mean just the :baz part
• Some parts of the Docker documentation use the term 'repository' to mean foo:8080/bar, but the documentation for docker tag claims that the repository is actually just foo:8080, and bar is the 'name part'

This makes some parts of the documentation confusing, particularly surrounding docker rmi and docker image prune.

The documentation should be updated to:

• Establish a clear convention on what the various parts of an image identifier are called, as well as the identifier as a whole (and don't forget to cover identifiers using digests)
• Add a page which explicitly documents this convention
• Ensure said convention is followed consistently everywhere (including in the output of the docker commands)

Start by reviewing the existing Docker documentation for docker tag, docker rmi, and docker image prune, using foo:8080/bar:baz and digest-based identifiers as examples. Define consistent names for each identifier component and the complete identifier, then add the convention page and audit documentation and command output for consistent usage.