Add line about possibly empty groups

Signed-off-by: Larsen, Steffen <steffen.larsen@intel.com>

sycl/doc/extensions/experimental/sycl_ext_oneapi_non_uniform_groups.asciidoc

@@ -357,6 +357,8 @@ _Returns_: A `range` representing the number of ballot-groups.
 NOTE: This will always return a `range` of 2, as there will always be two groups;
 one representing the group of work-items where the predicate was true and
 another representing the group of work-items where the predicate was false.
+It is possible for one of these groups to contain no work-items, if the
+predicate has the same value on all work-items.
 
 [source,c++]
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