Add missing documentation for cabal formatters

[fendor]

Fixes #4313

Adds documentation about the default formatters. I am not sure that's the correct location, though, to document the defaults.

[fendor]

Seems like I forgot to add the cabalFormattingProvider to vscode :see_no_evil: My mistake.

https://github.com/haskell/vscode-haskell/issues/1099

[fendor]

@michaelpj I think we should merge this as is, as it merely documents the status-quo