Improve formatting for record style GADT declarations

[alanz]

See https://ghc.haskell.org/trac/ghc/ticket/11028#comment:40

At the moment (f4ef254) a GADT declaration with record syntax is represented in haddock as

    CmmCondBranch :: {..} -> CmmNode O C
        cml_pred :: CmmExpr
        cml_true, cml_false :: !Label
        cml_likely :: Maybe Bool

for

    CmmCondBranch :: {              -- conditional branch
        cml_pred :: CmmExpr,
        cml_true, cml_false :: ULabel,
        cml_likely :: Maybe Bool    -- likely result of the conditional,
                                    -- if known
    } -> CmmNo O C

Change the haddock format to reflect the source format, by adding braces and moving the return type to the end.

[harpocrates]

Fixed in aa33be5. The latest GHC docs reflect it too: