Add documentation on invoke-only APIs

Bug: 22872577

Four Allocation related runtime API functions are not supposed to be
called from inside a kernel. Edit the documentation to clarify this
restriction.

Change-Id: I6766a216627fe485d2429c5455c02d0a151c68f8
diff --git a/api/rs_io.spec b/api/rs_io.spec
index eca2259..f10b58f 100644
--- a/api/rs_io.spec
+++ b/api/rs_io.spec
@@ -31,6 +31,10 @@
 description:
  Receive a new set of contents from the queue.
 #TODO We need better documentation.
+
+ This function should not be called from inside a kernel, or from any function
+ that may be called directly or indirectly from a kernel. Doing so would cause a
+ runtime error.
 test: none
 end:
 
@@ -42,6 +46,10 @@
 description:
  Send the contents of the Allocation to the queue.
 #TODO We need better documentation.
+
+ This function should not be called from inside a kernel, or from any function
+ that may be called directly or indirectly from a kernel. Doing so would cause a
+ runtime error.
 test: none
 end: