config: Split annotations restrictions into one-per-sentence

As recommended by Daniel [1], although I've adjusted his recommended
wording slightly and split the requirements into paragraphs.

[1]: #654 (comment)

Signed-off-by: W. Trevor King <wking@tremily.us>

config.md

@@ -391,15 +391,19 @@ If a hook returns a non-zero exit code, then an error is logged and the remainin
 
 **`annotations`** (object, OPTIONAL) contains arbitrary metadata for the container.
 This information MAY be structured or unstructured.
-Annotations MUST be a key-value map where both the key and value MUST be strings.
-While the value MUST be present, it MAY be an empty string.
+Annotations MUST be a key-value map.
+If there are no annotations then this property MAY either be absent or an empty map.
+
+Keys MUST be strings.
 Keys MUST be unique within this map.
 Keys MUST NOT be an empty string.
 Keys SHOULD be named using a reverse domain notation - e.g. `com.example.myKey`.
 Keys using the `org.opencontainers` namespace are reserved and MUST NOT be used by subsequent specifications.
-If there are no annotations then this property MAY either be absent or an empty map.
 Implementations that are reading/processing this configuration file MUST NOT generate an error if they encounter an unknown annotation key.
 
+Values MUST be strings.
+Values MAY be an empty string.
+
 ```json
 "annotations": {
     "com.example.gpu-cores": "2"