treewide: add warning comment to “boot” packages

This adds a warning to the top of each “boot” package that reads:

  Note: this package is used for bootstrapping fetchurl, and thus cannot
  use fetchpatch! All mutable patches (generated by GitHub or cgit) that
  are needed here should be included directly in Nixpkgs as files.

This makes it clear to maintainer that they may need to treat this
package a little differently than others. Importantly, we can’t use
fetchpatch here due to using <nix/fetchurl.nix>. To avoid having stale
hashes, we need to include patches that are subject to changing
overtime (for instance, gitweb’s patches contain a version number at
the bottom).

pkgs/development/interpreters/python/cpython/default.nix

@@ -36,6 +36,11 @@
 , enableOptimizations ? (!stdenv.isDarwin)
 }:
 
+# Note: this package is used for bootstrapping fetchurl, and thus
+# cannot use fetchpatch! All mutable patches (generated by GitHub or
+# cgit) that are needed here should be included directly in Nixpkgs as
+# files.
+
 assert x11Support -> tcl != null
                   && tk != null
                   && xorgproto != null