feat(graindoc): Allow doc comments on variants and record fields

[alex-snezhko]

Closes #992

provide record R {
  /**
   * A very special number
   */
  x: Number
}

provide enum E {
  /**
   * A thing
   */
  Thing
}

[phated]

Excited to look at this! I'm pretty sure you need to add tests now that @spotandjake added the test harness

[spotandjake]

This looks really good, I love how much it cleaner it makes the docs. I just had two little notes to keep the tables in line with the function tables but besides that it looks good.

Also after this is merged someone will need to update the website docs on this feature.

[peblair]

Looks good, but holding off on an approval until @spotandjake's comments are addressed

[phated]

I don't like the nested table so I proposed an alternative. Also had a question about levels and a possible bug (that we should fix and add a test for).

[phated]

I like the output a lot! I had just a couple comments on the implementation