scripts: Explain the difference between linux-headers and standard-headers

If you don't know it, it's hard to figure out the difference between
the linux-headers folder and the include/standard-headers folder.
So let's add a short explanation to clarify the difference.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>

scripts/update-linux-headers.sh

@@ -9,6 +9,22 @@
 #
 # This work is licensed under the terms of the GNU GPL version 2.
 # See the COPYING file in the top-level directory.
+#
+# The script will copy the headers into two target folders:
+#
+# - linux-headers/ for files that are required for compiling for a
+#   Linux host.  Generally we have these so we can use kernel structs
+#   and defines that are more recent than the headers that might be
+#   installed on the host system.  Usually this script can do simple
+#   file copies for these headers.
+#
+# - include/standard-headers/ for files that are used for guest
+#   device emulation and are required on all hosts.  For instance, we
+#   get our definitions of the virtio structures from the Linux
+#   kernel headers, but we need those definitions regardless of which
+#   host OS we are building for.  This script has to be careful to
+#   sanitize the headers to remove any use of Linux-specifics such as
+#   types like "__u64".  This work is done in the cp_portable function.
 
 tmpdir=$(mktemp -d)
 linux="$1"