Update comments in c9s repo files

This will help to explain what each of them are used for and why we need
both.

c9s-mirror.repo

@@ -1,6 +1,7 @@
 # These are the official c9s repos. They are slower to update, but contain older
 # versions of packages, which is useful when pinning for lack of a "coreos-pool"
-# equivalent. They are unused when no pinning is needed.
+# equivalent. When no pinning is needed you may find the compose repo URLs
+# defined in c9s.repo are quicker to get new content.
 
 [c9s-baseos-mirror]
 name=CentOS Stream 9 - BaseOS

c9s.repo

@@ -1,3 +1,9 @@
+# These are compose repo URLs that represent the latest composes in
+# CentOS Stream 9. Sometimes these repos get content a little faster
+# than the mirror repos defined in c9s-mirror.repo, but they won't
+# have multiple versions of packages, which make them not ideal when
+# needing to pin on older package versions.
+
 [c9s-baseos]
 name=CentOS Stream 9 - BaseOS
 baseurl=https://composes.stream.centos.org/production/latest-CentOS-Stream/compose/BaseOS/$basearch/os