Feature request: Syntax for admonitions (TIP/IMPORTANT/WARNING/etc.) markup

[m-renaud]

Several other markup languages support custom formatting of admonitions (see RST docs for example). They makes certain call-outs render in a format much easier to read (see Important, Tip, and Note markup in https://haskell-containers.readthedocs.io/en/latest/set.html). Can we support this in some capacity in Haddock.