Comments on instances
Ignore-this: 8698501a4b1b9d3b6ebbc7ff6893b3dc

This was a little trickier than I thought, since we need to match up instances
from the renamed syntax with instances represented by InstEnv.Instance. This is
due to the current design of Haddock, which matches comments with declarations
from the renamed syntax, while getting the list of instances of a class/family
directly using the GHC API.

The comments are rendered to the right of the instance head in the HTML output.

Note:

- No change to the .haddock file format.

- Works for both class and type family instances.

- Works for normal user-written instances only. No comments are added on
  derived or TH-generated instances.
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This patch contains the following changes: