List supported architectures for StackMap section and related intrinsics

Not having this documented led to some confusion in a recent review thread.

llvm-svn: 242441
diff --git a/llvm/docs/StackMaps.rst b/llvm/docs/StackMaps.rst
index dbdf78f..5bdae38 100644
--- a/llvm/docs/StackMaps.rst
+++ b/llvm/docs/StackMaps.rst
@@ -499,3 +499,13 @@
 transformations must not substitute the alloca with any intervening
 value. This can be verified by the runtime simply by checking that the
 stack map's location is a Direct location type.
+
+
+Supported Architectures
+=======================
+
+Support for StackMap generation and the related intrinsics requires 
+some code for each backend.  Today, only a subset of LLVM's backends 
+are supported.  The currently supported architectures are X86_64, 
+PowerPC, and Aarch64.
+
diff --git a/llvm/docs/Statepoints.rst b/llvm/docs/Statepoints.rst
index b1d1ed8..0d91f1b 100644
--- a/llvm/docs/Statepoints.rst
+++ b/llvm/docs/Statepoints.rst
@@ -699,6 +699,12 @@
 llvmdev mailing list for suggestions.
 
 
+Supported Architectures
+=======================
+
+Support for statepoint generation requires some code for each backend.
+Today, only X86_64 is supported.  
+
 Bugs and Enhancements
 =====================