MigrateProcessInterface documents ProcessPluginBase behaviour

Problem/Motivation

\Drupal\migrate\Plugin\MigrateProcessInterface documents that multiple entry points can be used in the process plugin, but that is logic that lives on the \Drupal\migrate\ProcessPluginBase class.

Proposed resolution

Move the documentation where the mentioned logic is implemented, i.e. \Drupal\migrate\ProcessPluginBase.

Remaining tasks

Write a patch.

User interface changes

N.A.

API changes

N.A.

Data model changes

N.A.