ci_scripts: deprecated: update comments

Make the explanation for the deprecated.txt file's purpose more
explicit.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>

Author: threexc

ci_scripts/deprecated.txt

@@ -3,9 +3,10 @@
 
 # SPDX-License-Identifier: Apache-2.0
 
-# List of deprecated packages.
-# When we deprecated a package, we add it here, to check if upstream is still
-# building it
+# List of deprecated packages. Deprecated packages are those which are now built
+for riscv64 platforms by the upstream projects, so we don't need to maintain a
+build workflow for it ourselves. Previously-built wheels should remain in the
+RISE package registry.
 
 aiohttp
 blake3