[GC docs] Fill in description of the statepoint-example GC strategy

llvm-svn: 230572
diff --git a/llvm/docs/Statepoints.rst b/llvm/docs/Statepoints.rst
index 5b58469..e1e7d9b 100644
--- a/llvm/docs/Statepoints.rst
+++ b/llvm/docs/Statepoints.rst
@@ -322,6 +322,7 @@
 side effects since it is just a way to extract information about work
 done during the actual call modeled by the ``gc.statepoint``.
 
+.. _statepoint-stackmap-format:
 
 Stack Map Format
 ================