[SYCL][Doc] Add leader to GroupAlgorithms (#1297)

Signed-off-by: John Pennycook <[email protected]>

Author: Pennycook

sycl/doc/extensions/GroupAlgorithms/SYCL_INTEL_group_algorithms.asciidoc

@@ -62,6 +62,7 @@ The extension introduces the following functions:
 - +any_of+
 - +all_of+
 - +none_of+
+- +leader+
 - +broadcast+
 - +reduce+
 - +exclusive_scan+
@@ -181,6 +182,17 @@ It is undefined behavior for any of these functions to be invoked within a +para
 |Return +true+ if _pred_ returns +true+ for no element in the range [_first_, _last_). _first_, _last_ and _pred_ must be the same for all work-items in the group.
 |===
 
+==== Elect
+
+The +leader+ function provides a mechanism to _elect_ a single work-item as the leader of the group, commonly in order to execute a task once per group.
+
+|===
+|Function|Description
+
+|+template <typename Group> bool leader(Group g);+
+|Return +true+ for exactly one work-item in the group, if the calling work-item is the elected leader of the group.  Every call to +leader+ with the same group _g_ must elect the same work-item.
+|===
+
 ==== Collectives
 
 In this section, the meaning of "exclusive scan" and "inclusive scan" are as defined in Sections 29.8.7 and 29.8.8 of the {cpp}17 specification, respectively.