top-level: Deprecate top-level {build,host,target}Platform

I don't know when we can/should remove them, but this at least gets
people to stop using them. The preferred alternatives also date back to
17.09 so writing forward-compatable code without extra conditions is
easy.

Beginning with these as they are the least controversial.

doc/cross-compilation.xml

@@ -47,13 +47,9 @@
 
    <para>
     In Nixpkgs, these three platforms are defined as attribute sets under the
-    names <literal>buildPlatform</literal>, <literal>hostPlatform</literal>,
-    and <literal>targetPlatform</literal>. All three are always defined as
-    attributes in the standard environment, and at the top level. That means
-    one can get at them just like a dependency in a function that is imported
-    with <literal>callPackage</literal>:
-<programlisting>{ stdenv, buildPlatform, hostPlatform, fooDep, barDep, .. }: ...buildPlatform...</programlisting>
-    , or just off <varname>stdenv</varname>:
+    names <literal>buildPlatform</literal>, <literal>hostPlatform</literal>, and
+    <literal>targetPlatform</literal>. They are always defined as attributes in
+    the standard environment. That means one can access them like:
 <programlisting>{ stdenv, fooDep, barDep, .. }: ...stdenv.buildPlatform...</programlisting>
     .
    </para>