pkgs/top-level: rewrite some outdated comments

This removes all specific references to pkgsCross or pkgsi686Linux, because
they have become outdated with the addition of many more package sets.

pkgs/top-level/default.nix

@@ -101,22 +101,18 @@ in let
   config = lib.showWarnings configEval.config.warnings configEval.config;
 
   # A few packages make a new package set to draw their dependencies from.
-  # (Currently to get a cross tool chain, or forced-i686 package.) Rather than
-  # give `all-packages.nix` all the arguments to this function, even ones that
-  # don't concern it, we give it this function to "re-call" nixpkgs, inheriting
-  # whatever arguments it doesn't explicitly provide. This way,
-  # `all-packages.nix` doesn't know more than it needs too.
+  # Rather than give `all-packages.nix` all the arguments to this function,
+  # even ones that don't concern it, we give it this function to "re-call"
+  # nixpkgs, inheriting whatever arguments it doesn't explicitly provide. This
+  # way, `all-packages.nix` doesn't know more than it needs to.
   #
   # It's OK that `args` doesn't include default arguments from this file:
   # they'll be deterministically inferred. In fact we must *not* include them,
   # because it's important that if some parameter which affects the default is
   # substituted with a different argument, the default is re-inferred.
   #
-  # To put this in concrete terms, this function is basically just used today to
-  # use package for a different platform for the current platform (namely cross
-  # compiling toolchains and 32-bit packages on x86_64). In both those cases we
-  # want the provided non-native `localSystem` argument to affect the stdenv
-  # chosen.
+  # To put this in concrete terms, we want the provided non-native `localSystem`
+  # and `crossSystem` arguments to affect the stdenv chosen.
   #
   # NB!!! This thing gets its `config` argument from `args`, i.e. it's actually
   # `config0`. It is important to keep it to `config0` format (as opposed to the

pkgs/top-level/stage.nix

@@ -176,12 +176,7 @@ let
       ((config.packageOverrides or (super: {})) super);
 
   # Convenience attributes for instantitating package sets. Each of
-  # these will instantiate a new version of allPackages. Currently the
-  # following package sets are provided:
-  #
-  # - pkgsCross.<system> where system is a member of lib.systems.examples
-  # - pkgsMusl
-  # - pkgsi686Linux
+  # these will instantiate a new version of allPackages.
   otherPackageSets = self: super: {
     # This maps each entry in lib.systems.examples to its own package
     # set. Each of these will contain all packages cross compiled for