os/linux/extend/ENV/super: add comment.

Library/Homebrew/extend/os/linux/extend/ENV/super.rb

@@ -32,6 +32,9 @@ module Superenv
     m4_path_deps = ["libtool", "bison"]
     self["M4"] = "#{HOMEBREW_PREFIX}/opt/m4/bin/m4" if deps.any? { m4_path_deps.include?(_1.name) }
 
+    # Pointer authentication and BTI are hardening techniques most distros
+    # use by default on their packages. arm64 Linux we're packaging
+    # everything from scratch so the entire dependency tree can have it.
     append_to_cccfg "b" if Hardware::CPU.arch == :arm64 && DevelopmentTools.gcc_version("gcc") >= 9
   end