man containers-registries.conf: clarify mirror association

Clarify that a `[[registry.mirror]]` is associated only with the
previous `[[registry]]`.

Fixes: #1523
Signed-off-by: Valentin Rothberg <vrothberg@redhat.com>

docs/containers-registries.conf.5.md

@@ -99,7 +99,7 @@ as-is. But other settings like insecure/blocked/mirrors will be applied to match
 
 `mirror`
 : An array of TOML tables specifying (possibly-partial) mirrors for the
-`prefix`-rooted namespace.
+`prefix`-rooted namespace (i.e., the previous registry).
 
 The mirrors are attempted in the specified order; the first one that can be
 contacted and contains the image will be used (and if none of the mirrors contains the image,
@@ -236,6 +236,12 @@ location = "example-mirror-0.local/mirror-for-foo"
 [[registry.mirror]]
 location = "example-mirror-1.local/mirrors/foo"
 insecure = true
+
+[[registry]]
+location = "registry.com"
+
+[[registry.mirror]]
+location = "mirror.registry.com"
 ```
 Given the above, a pull of `example.com/foo/image:latest` will try:
     1. `example-mirror-0.local/mirror-for-foo/image:latest`
@@ -244,6 +250,8 @@ Given the above, a pull of `example.com/foo/image:latest` will try:
 
 in order, and use the first one that exists.
 
+Note that a mirror is associated only with the previous registry.  Pulling `registry.com/image:latest` will hence only reach out to `mirror.registry.com` and not use the one associated to the other registry.
+
 ## VERSION 1 FORMAT - DEPRECATED
 VERSION 1 format is still supported but it does not support
 using registry mirrors, longest-prefix matches, or location rewriting.