webp-container-spec.txt: clarify 'VP8 '/'XMP ' fourccs

Note the fourth character is an ASCII space to avoid confusion.
Also update the FourCC definition to mention that they're case
sensitive.

Change-Id: I87d0cbcf06e7c25bf2b33419b4c8c891adb82a8a

doc/webp-container-spec.txt

@@ -84,7 +84,8 @@ _uint32_
 _FourCC_
 
 : A _FourCC_ (four-character code) is a _uint32_ created by concatenating four
-  ASCII characters in little-endian order.
+  ASCII characters in little-endian order. This means 'aaaa' (%x61.61.61.61) and
+  'AAAA' (%x41.41.41.41) are treated as different _FourCCs_.
 
 _1-based_
 
@@ -203,6 +204,8 @@ VP8 data: _Chunk Size_ bytes
 
 : VP8 bitstream data.
 
+Note the fourth character in the 'VP8 ' fourcc is an ASCII space (%x20).
+
 The VP8 bitstream format specification can be found at [VP8 Data Format and
 Decoding Guide][vp8spec]. Note that the VP8 frame header contains the VP8 frame
 width and height. That is assumed to be the width and height of the canvas.
@@ -714,6 +717,8 @@ XMP Metadata: _Chunk Size_ bytes
 
 : image metadata in XMP format.
 
+Note the fourth character in the 'XMP ' fourcc is an ASCII space (%x20).
+
 Additional guidance about handling metadata can be found in the
 Metadata Working Group's [Guidelines for Handling Metadata][metadata].