Constructor and pattern synonym argument docs (#709)

* Support Haddocks on constructor arguments

This is in conjunction with https://phabricator.haskell.org/D4094.
Adds support for rendering Haddock's on (non-record) constructor
arguments, both for regular and GADT constructors.

* Support haddocks on pattern synonym arguments

It appears that GHC already parsed these - we just weren't using them.
In the process of doing this, I tried to deduplicate some code around
handling patterns.

* Update the markup guide

Add some information about the new support for commenting constructor
arguments, and mention pattern synonyms and GADT-style constructors.

* Overhaul LaTeX support for data/pattern decls

This includes at least

  * fixing several bugs that resulted in invalid LaTeX
  * fixing GADT data declaration headers
  * overhaul handling of record fields
  * overhaul handling of GADT constructors
  * overhaul handling of bundled patterns
  * add support for constructor argument docs

* Support GADT record constructors

This means changes what existing HTML docs look like.

As for LaTeX, looks like GADT records were never even supported. Now they are.

* Clean up code/comments

Made code/comments consistent between the LaTeX and XHTML backend
when possible.

* Update changelog

* Patch post-rebase regressions

* Another post-rebase change

We want return values to be documentable on record GADT constructors.

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