…nks.

…rameter.

- Renaming registry-version paramater to be consistent.
- Adding description of features parameter.
- Adding a distinct examples section.

…ditional, unspecified qualifeirs must be tolerated.

… of the registry_version qualifier should be percent-encoded.

Revive package-url#245 by adding the
following changes:

- `registry_url` -> `repository_url`
- `registry_version` -> `port_revision`
- Remove percent escaping from `repository_url` to be more consistent
  with other uses of `repository_url` in the purl spec

Co-authored-by: Michael B. Price <michael.b.price.dev@gmail.com>

Add optional `repository_revision` so that mistakes in `port_revision` and
`version` can be accounted for. Not relevant for filesystem registries
or overlay ports because that gives no further external traceability.

Along with this, describe the filesystem registries and overlay port
cases.

Extend the port overlay or filesystem registry example with port
revision.

Add an example for additional qualifiers.