webp-container-spec: clarify background color note
replace ...a transparency value (alpha)... with clearer text
based on comments from:
@@ -400,8 +400,8 @@ Background Color: 32 bits (_uint32_)
**Note**:
- * Background color MAY contain a transparency value (alpha), even if the
- _Alpha_ flag in [VP8X chunk](#extended_header) is unset.
+ * Background color MAY contain a non-opaque alpha value, even if the _Alpha_
+ flag in [VP8X chunk](#extended_header) is unset.
* Viewer applications SHOULD treat the background color value as a hint, and
are not required to use it.
