Update docs to reflect new requirements and attributes

- recent systems need `libtinfo6` instead of `libtinfo5`
- python2 is no longer used and the `python` package is no longer available

Author: avdv

docs/haskell-use-cases.rst

@@ -54,9 +54,17 @@ toolchain resolution based on the target platform)::
   )
 
   rules_haskell_toolchains(
+      dist = {
+          "<arch>": "abc", # Select a concrete dist available for the <arch>
+      },
+      variant = {
+          "<arch>": "abc", # Select a concrete variant available for the <arch> and <dist>
+      }
       version = "X.Y.Z", # Any GHC version
   )
 
+*Note*: if no `dist` attribute is specified, we pick a Debian based binary
+        distribution on Linux by default.
 
 The compiler can also be pulled from Nixpkgs_, a set of package
 definitions for the `Nix package manager`_. Pulling the compiler from

docs/haskell.rst

@@ -30,14 +30,17 @@ On a Unix system you will need the following tools installed.
 * ``gcc``
 * ``libffi``
 * ``libgmp``
-* ``libtinfo5``
+* ``libtinfo6``
 * ``make``
-* ``python3`` (``python`` also needs to be available in ``$PATH``. Depending on your distribution, this might require installing the ``python`` meta-package, which might use Python 2 or 3, ``rules_haskell`` works with both.)
+* ``python3``
 * JDK (e.g., OpenJDK 8 or 11)
 
 On Ubuntu you can obtain them by installing the following packages. ::
 
-  build-essential libffi-dev libgmp-dev libtinfo5 libtinfo-dev python python3 openjdk-11-jdk
+  build-essential libffi-dev libgmp-dev libtinfo6 libtinfo-dev python3 openjdk-11-jdk
+
+*Note*: on older Unix systems, with older GHC versions you might need to install
+        ``libtinfo5`` instead of ``libtinfo6`` and ``libnuma-dev`` additionally.
 
 On Windows you will need.