Clarify Sphinx-Needs type names in docs

[Mirochill]

Summary

• update filtering examples to use the Sphinx-Needs type names testfile, testsuite, and testcase
• add a note distinguishing those type names from the hyphenated directives test-file, test-suite, and test-case
• fix the test-case directive page copy/paste text so it describes test-case and the testcase type
• update the needlist examples to compare the numeric cases field directly

Fixes #131

Validation

• git diff --check -- docs/filter.rst docs/directives/test_file.rst docs/directives/test_suite.rst docs/directives/test_case.rst
• Read the Docs build succeeded: https://sphinx-test-reports--136.org.readthedocs.build/en/136/
• Not run locally: tests (documentation-only change)