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

llvm-svn: 230572

docs/GarbageCollection.rst

@@ -386,8 +386,8 @@ greater performance impact since pointer reads are more frequent than writes.
 
 .. _plugin:
 
-Built In Collectors
+Built In GC Strategies
-====================
+======================
 
 LLVM includes built in support for several varieties of garbage collectors.  
 
@@ -501,7 +501,19 @@ The Statepoint Example GC
   F.setGC("statepoint-example");
 
 This GC provides an example of how one might use the infrastructure provided 
-by ``gc.statepoint``.  
+by ``gc.statepoint``. This example GC is compatible with the 
+:ref:`PlaceSafepoints` and :ref:`RewriteStatepointsForGC` utility passes 
+which simplify ``gc.statepoint`` sequence insertion. If you need to build a 
+custom GC strategy around the ``gc.statepoints`` mechanisms, it is recommended
+that you use this one as a starting point.
+
+This GC strategy does not support read or write barriers.  As a result, these 
+intrinsics are lowered to normal loads and stores.
+
+The stack map format generated by this GC strategy can be found in the 
+:ref:`stackmap-section` using a format documented :ref:`here 
+<statepoint-stackmap-format>`. This format is intended to be the standard 
+format supported by LLVM going forward.
 
 
 Custom GC Strategies

docs/Statepoints.rst

@@ -322,6 +322,7 @@ A ``gc.relocate`` is modeled as a ``readnone`` pure function.  It has no
 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
 ================