Document primitive shape principle axis (e.g. how is orientation defined for a cylinder?)

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/61)) by John Hsu (Bitbucket: [hsu](https://bitbucket.org/%7B0a186eae-abf0-4514-a951-23db5eccc286%7D/), GitHub: [hsu](https://github.com/hsu)).**

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Start by reviewing the existing documentation for primitive shapes and the archived issue report. Determine how the orientation axis is defined for shapes such as cylinders, then document the agreed principle and examples; the issue does not identify a file, test, or more specific acceptance criteria.

Written by the indexing model from the issue text.