Clean up language around tag listing and repository naming

Tag listing should mention the endpoints and external method
relying on content addressability.
Image repositories should not be referred to as indexes, but
rather as image repositories.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

proposals/distribution.md

@@ -103,11 +103,11 @@ The following entries should be added to the [scope table][scope]:
 * “Specifying a distribution method”.
     This entry replaces part of the previous “Creating Reference spec for optional DNS based naming & distribution” and “Standardizing on a particular Distribution method” entries.
 
-    Retrieving image indexes covers the current “tag listing” (e.g. “what named manifests are in `library/busybox`?”), because tags are entries in the image format's [`manifests` array][manifests].
-    Other tag-listing endpoints needed for backwards-compatibility are therefore in scope as well.
+    Tag-listing (e.g. “what named manifests are in `library/busybox`?”) endpoints are in scope and required for backwards compatibility with clients.
+    External methods for listing tags remain compatible through fetching by content address, but are out of scope for definition.
 
-    Grouping image indexes in repositories is considered part of distribution policy or content management, which are out of scope for this entry's per-image action.
-    For example, “what images are under `library/`?” is out of scope for this project.
+    Grouping image repository names is considered part of distribution policy or content management, which are out of scope.
+    For example, “which image repositories are under `library/`?” is out of scope for this project.
 
     * What: Specifying a distribution method
     * In/Out/Future: In scope