docs: improve and increase coverage of in-code documentation

[nokome]

Once this is done, set tslint.json to:

{
  "extends": "tslint-config-standard",
  "rules" : {
    "completed-docs": true
  }
}

[nokome]

I've gone and done this for most files but will leave the Python related file to you @beneboy . They currently have /* tslint:disable: completed-docs */ at the top. For a lot of the helper functions (e.g. in PythonParser.ts) whose names are already pretty self documenting you could just put a tslint disable block around them?

 `/* tslint:disable: completed-docs */`
...
 `/* tslint:enable: completed-docs */`

[stencila-ci]

🎉 This issue has been resolved in version 0.12.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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