Clarify CompareGuid change

* Update guid.c to display correct output code in description
* Add warning about failure in ABI README

Signed-off-by: Callum Farmer <gmbr3@opensuse.org>

docs/README.ABI.md

@@ -5,9 +5,13 @@ versions
 
    This currently includes the following functions:
    - ReallocatePool
+
    - CopyMem
+
    - CompareGuid
    
+     **Will break at runtime - likely causing opposite output to expected**
+
 2) Applications which haven't been updated in a while may also be missing the
 required `.rodata` section which contains needed read-only data which will prevent
 any binary missing this section from running correctly

lib/guid.c

@@ -134,7 +134,7 @@ Arguments:
     Guid2       - guid to compare
 
 Returns:
-    = 0     if Guid1 == Guid2
+    = 1     if Guid1 == Guid2
 
 --*/
 {