[editorial] rework paragraph about ReferenceBlock 0

ebml_matroska.xml

@@ -207,7 +207,7 @@ This is used to reference other frames necessary to decode this frame.
 The relative value **SHOULD** correspond to a valid `Block` that this `Block` depends on.
 Historically, Matroska Writers didn't write the actual `Block(s)` that this `Block` depends on, but they did write *some* `Block(s)` in the past.
 
-The value "0" **MAY** also be used to signify that this `Block` cannot be decoded on its own, but without knowledge of which `Block` is necessary. In this case, other `ReferenceBlock` Elements **MUST NOT** be found in the same `BlockGroup`.
+The value "0" **MAY** also be used to signify that this `Block` cannot be decoded on its own, but the necessary reference `Block(s)` is unknown. In this case, other `ReferenceBlock` Elements **MUST NOT** be found in the same `BlockGroup`.
 
 If the `BlockGroup` doesn't have a `ReferenceBlock` element, then the `Block` it contains can be decoded without using any other `Block` data.</documentation>
     <extension type="webmproject.org" webm="1"/>