clarify: add that binaryData is base64-encoded

Author: Shegox

content/en/docs/concepts/configuration/configmap.md

@@ -43,7 +43,7 @@ Kubernetes objects that have a `spec`, a ConfigMap has `data` and `binaryData`
 fields. These fields accept key-value pairs as their values.  Both the `data`
 field and the `binaryData` are optional. The `data` field is designed to
 contain UTF-8 byte sequences while the `binaryData` field is designed to
-contain binary data.
+contain binary data as base64-encoded strings.
 
 The name of a ConfigMap must be a valid
 [DNS subdomain name](/docs/concepts/overview/working-with-objects/names#dns-subdomain-names).