A couple of features of failCommand are not tested in jstests/core/testing/failcommand_failpoint.js. One such example is failInternalCommands. Add tests to cover all untested features. The only documentation available about failCommand in docs/fail_points.md points to this test file as a source of truth for demonstrative examples of its features, so adding test coverage is akin to also adding documentation for these features; we may want to consider adding proper documentation for the features under docs/fail_points.md.