Clarify requirements for label value syntax

Author: veyu

content/en/docs/concepts/overview/working-with-objects/labels.md

@@ -52,7 +52,10 @@ If the prefix is omitted, the label Key is presumed to be private to the user. A
 
 The `kubernetes.io/` and `k8s.io/` prefixes are reserved for Kubernetes core components.
 
-Valid label values must be 63 characters or less and must be empty or begin and end with an alphanumeric character (`[a-z0-9A-Z]`) with dashes (`-`), underscores (`_`), dots (`.`), and alphanumerics between.
+Valid label value:
+* must be 63 characters or less (cannot be empty),
+* must begin and end with an alphanumeric character (`[a-z0-9A-Z]`),
+* could contain dashes (`-`), underscores (`_`), dots (`.`), and alphanumerics between.
 
 For example, here's the configuration file for a Pod that has two labels `environment: production` and `app: nginx` :