Add documentation for taxonomy terms source plugins

Problem/Motivation

Following the example of #2937989: Support multiple node types in node source plugins we'd like to add documentation and examples to migration source plugins, so they're easier to work with.

This issue will cover plugins for taxonomy terms:

• core/modules/taxonomy/src/Plugin/migrate/source/d6/Term.php
• core/modules/taxonomy/src/Plugin/migrate/source/d6/TermLocalizedTranslation.php
• core/modules/taxonomy/src/Plugin/migrate/source/d7/Term.php
• core/modules/taxonomy/src/Plugin/migrate/source/d7/TermEntityTranslation.php
• core/modules/taxonomy/src/Plugin/migrate/source/d7/TermLocalizedTranslation.php
• core/modules/taxonomy/src/Plugin/migrate/source/d7/TermTranslation.php

Remaining tasks

1. Review <- we are here
2. Address the feedback
3. Commit