[GlobalISel] Update the documentation of abd.

The abd nodes do not perform the same as abs(x-y), although they are often
mistaken to do so. They extend into a larger bit size before performing the sub
/ abs. Update the description of the instruction to avoid misunderstandings.

Author: davemgreen

llvm/docs/GlobalISel/GenericOpcode.rst

@@ -504,7 +504,7 @@ undefined.
 G_ABDS, G_ABDU
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-Compute the absolute difference (signed and unsigned), e.g. abs(x-y).
+Compute the absolute difference (signed and unsigned), e.g. trunc(abs(ext(x)-ext(y)).
 
 .. code-block:: none